* 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.

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.

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.

The first variable can be unkeyed and the provider will sub in the
variable name 'name'. Additionally, the built-in templates and
documentation have been updated so that 'appid' is now 'name' and the
alternative commands are shown.

This totally breaks compatibility with rebar2, and maybe it shouldn't
have.