Add support for dep locking via rebar_lock_deps_plugin

* If a rebar.config.lock file is present, it will be used with the
  exception of the prepare_release target and its friends.

* The allclean target is enhanced to detect with the rebar in play
  requires a -r flag or not to clean all deps.

* Fix bad copy/paste hard code in newly added devrel target

* Add prepare_release target with relx integration

* Generated rebar.config now includes {use_lock_deps, X}. X defaults to
  false for non-active projects and true otherwise. It is also possible
  to specify a branch or tag of the rebar_lock_deps_plugin here.

* Refactor cleanup of generated rebar.config.script. Incremented version
  to 2. Adds support for optionally including rebar_lock_deps_plugin.