* allow plugins to print help message for implemented commands
* append core rebar.config options to common 'rebar help' message

This allows `rebar ct suites=abc` to consider all suites when
you have a rebar setup with multiple sub_dirs.

Previously, rebar halted after it could not find the suite in the
first dir. But the suite might be present in another dir (when
sub_dirs contains multiple dirs).

This commit makes it so instead of halting, it prints a warning and
continues with looking for the suite in the other `sub_dir`s.

Note -- This uses try/catch to cause the test to be skipped because
otherwise I needed to adjust the return values of 4 functions, the
code path is pretty deeply nested here. Otherwise the whole call chain
needed to be adjusted for this return value:

`run_test -> make_cmd -> get_suites -> find_suite_path`

IMHO, I think for exceptional cases like this it is fine to use throw;
specially since only the {skip} is catched and nothing else.

When rebar ct executes with its default common test directory of "test",
it will generate a hardcoded "logs" directory in every application with
a test directory present, causing an overlap with eunit's test framework
so even test directories with only eunit tests will be processed by ct.

Not every system under test can be run with long names, and this makes
rebar's common_test support useless in those environments, as it
currently uses long names (test@hostname.domain) by default, without
recourse to change them.

This patch adds support for a {ct_use_short_names, boolean()} config
variable, which allows the user to specify whether short or long names
are required.

This patch allows users to specify the directory in which common_test
source files can be found. Most common_test suites are integration,
rather than unit tests and keeping the sources apart from test sources
for other frameworks such as eunit and PropEr is a useful feature.

This patch adds a `ct_extra_params` option to rebar.config, the value
of which is appended to the shell command when executing common test.

This change adds support for executing ct test runs based on test
specificiations, which was missing previously. The rebar_ct module
now looks for any number of files with a name ending in `test.spec`
and if it finds one or more, passes these after the `-spec` argument
to ct_run instead of explicitly configuring the config, user config
and coverage config variables.

When no specifications are found, then the module behaves as it did
before this change, and both the ct1 and (new) ct2 integration tests
appear to show this is a backwards compatible patch.

Add flags to rebar_utils:sh to control output and error handling.
Replace calls to os:cmd with calls to rebar_utils:sh.

rebar_utils:sh/2 terminates on error and does not return an error code
as expected.