* bump bbmustache to version 1.6.0

* Use alternative bbmustache brackets in templates

This fixes the problems where literal nested tuples (`{{a,b},c}` or
`{{a,b}}`) would blow up and spaces needed to be inserted.

People are starting to expect these niceties! It's also easier to drop
the config when you know it's there than add it when you don't know it
exists.

fixes an issue when shell is terminated with an error "Bus error: 10",
on attempt to run rebar shell with verbose logging

Gets in line with more common usage of the wording, as discussed in
issue #1421 and
https://www.reddit.com/r/erlang/comments/6sd01p/help_terribly_frustrated_with_the_erlang_build/

This is done through 3 main change groups:

- replacing `~s` by `~ts` in format strings, so that strings that
  contain unicode are properly printed rather than crashing
- adding the `unicode` argument to all function of the `re` module to
  ensure transformations on strings containing unicode data are valid
  instead of crashing (see issue #1302)
- replacing `ec_cnv:to_binary/1` and `ec_cnv:to_list/1` with matching
  functions in `rebar_utils`.

The last point has been done, rather than modifying and updating erlware
commons, because binary and list conversions can be a contentious
subject. For example, if what is being handled is actually bytes from a
given binary stream, then forcing a byte-oriented interpretation of the
data can corrupt it. As such, it does not appear safe to modify erlware
commons' conversion functions since it may not be safe for all its
users.

Instead, rebar3 reimplements a subset of them (only converting
atoms and chardata, ignoring numbers) with the explicit purpose of
handling unicode string data.

Tests were left as unchanged as possible. This may impact the ability to
run rebar3's own suites in a unicode path, but respects a principle of
least change for such a large patch.

The default license created with rebar3 templates is a BSD 3 clause
license. However the .app.src file generated do not include the license
there.

This adds the license type, fixing #1447.

This family of IDE with Erlang plugin is already quite popular.

Used a nonexisting option instead of a correct one

This patch adds support for short options in the eunit provider.

To prevent future mistakes like the broken string introduced in
cf66dfd6ba, put opening and closing " on separate lines in each
multi-line lopts string. This also allows for cleaner (smaller) diffs
when removing/adding a flag.

cf66dfd6ba introduced a syntax error in the modified lopts string.
Without the fix, you'll provoke a shell error when trying to complete
`rebar3 ct --<something>`.

Issues formerly were avoided by quoting atoms. Unquoting them created
unbalanced tags (`{{{name}}`) which were mistakes. This patch inserts
spaces to unconfused bbmustache.

hex.pm is now using maintainers instead of contributors in metadata.
Templates should be updated to reflect it.

Some templates like OTP apps are usable within releases, so the text
does remain a bit vague, but hopefully less than before.