- Uses the code at https://github.com/tsloughter/rebar_alias and brings
  it within rebar3
- adds safety checks to prevent redefining built-in commands or obvious
  circular dependencies between commands (indirect circular deps are
  still possible)
- adds tests
- adds a systest to ensure no clash with the existing plugin

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.

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.