Abuse error_handler to get free metacalls in r3

This uses the very risky '$handle_undefined_function'/2 export from the
r3 and rebar_agent modules to allow meta-calls that can support plugins
and all other rebar3 extensions.

This is nasty but very tempting. Currently we only support:

- r3:do(Command)
- r3:do(Namespace, Command)

There is currently no way to pass arguments to the function such that we
can, for example, run cover analysis or tests on a subset of suites.

With the new abuse of '$handle_undefined_function'/2, we can detect the
unused commands (since they are not exported) and re-route them:

- r3:Command()
- r3:Command("--args=as a string")
- r3:Command(Namespace, "--args=as a string")

Of course, in doing so, we make it impossible to use the 'do' provider
(as in 'rebar3 do ct -c, cover') since the 'do' function is already
required for things to work.

Since the previous function had very strict guards, we can, without
conflict, add manual overrides that simulate the meta-calls fine.

Sample run:
https://gist.github.com/ferd/2c06d59c7083c146d25e4ee301de0073

Fix typo, occured -> occurred

Bump relx to 3.22.4

Typo: seperate -> separate

- [OSX tests added to CI](https://github.com/erlang/rebar3/pull/1471),
joining linux and windows 10
- [upgrade relx to 3.22.3](https://github.com/erlang/rebar3/pull/1518),
[fixing multiple
issues](https://github.com/erlware/relx/releases/tag/v3.22.3)
- [Fixing eunit errors in 19.3 when `init_per_*` isn't paired with
`end_per_*`](https://github.com/erlang/rebar3/pull/1516)
- [Allow single test spec in
ct_opts](https://github.com/erlang/rebar3/pull/1506)
- [Properly warn when missing inets at
runtime](https://github.com/erlang/rebar3/pull/1505)
- [rebar_utils:escape_chars handles quotes in
paths](https://github.com/erlang/rebar3/pull/1504)
- [Fix typo in help text for upgrade
command](https://github.com/erlang/rebar3/pull/1503)
- [change package version warnings to debug
level](https://github.com/erlang/rebar3/pull/1502)
- [Path-type sensitive .cmd creation for
escripts](https://github.com/erlang/rebar3/pull/1500)
- [Windows escripts get cmd
autogenerated](https://github.com/erlang/rebar3/pull/1494)
- [REBAR_CONFIG impacts config file for project root
only](https://github.com/erlang/rebar3/pull/1497)
- [Fix wildcard usage in rebar3
clean](https://github.com/erlang/rebar3/pull/1498)
- [Bump cth_readable to
1.2.4](https://github.com/erlang/rebar3/pull/1496) (fixing some
erroneous reports when suites get skipped)
- [Add respect for src_dirs option in app_discover and
app_info](https://github.com/erlang/rebar3/pull/1486)
- [Add support for global plugin-defined
templates](https://github.com/erlang/rebar3/pull/1473)
- [catch badarg exception from eunit in
`rebar_prv_eunit`](https://github.com/erlang/rebar3/pull/1472)

update travis.yml to test on osx

upgrade relx to 3.22.3

eunit in 19.3 errors if `init_per_*` isn't paired with `end_per_*`

86e883b8d8 always returns the full length SHA,
therefore when using a dependency having the short SHA,
it'll always consider that the SHAs are different,
hence it'll alway return true for .

minor fix: name of internal function

Revert "List 'lock' command in man page"

List 'lock' command in man page

Allow single test spec in ct_opts

The option {spec,Specs} is allowed in ct_opts, but
rebar_prv_common_test:test_dirs did not take into account that Specs
could also be a string only, i.e. not a list of strings.

Properly warn on missing rebar3 deps: add inets

rebar_utils:escape_chars handles quotes

rebar_file_utils:cp_r uses rebar_utils:escape_chars to ensure that the file names are safe to use, but it doesn't escape double and single quotes. If the file name includes those characters, they disappear when the shell processes them and we get "file not found" errors.

The main culprit here is eunit, that creates reports whose names are `TEST-file_"myfile.app".xml`, and I wish it didn't but I think escape_chars should still do its job all the way.

Fix typo in help text for upagrde command

change package version warning to debug

Path-type sensitive .cmd creation

Windows escripts get cmd autogenerated

More fixes for bad calls to find_apps

REBAR_CONFIG impacts file project root only

Fix wildcard usage in rebar3 clean

The wildcard usage could fail on some OSes by being passed directly and
just not finding libs and erroring out

Bump cth_readable to 1.2.4

Fixes an issue where some skipped test suites could end up misreported
as the previous suite. See
https://github.com/ferd/cth_readable/issues/10

Add respect for src_dirs option in app_discover and app_info

This also patches up a problem for hooks when dealing with directories
with spaces in them, and reduces complexity of bootstrap file.