There are historical reasons that the `plugin` command was `plugin_new`, now those are no longer applicable, we should remove the naming edge case from the project. This PR is based off of comments from #11176
ATP Railties
When we removed script/rails and introduced bin/rails, we accidentally
introduced a regression. If you install Rails 4 as a gem, then try to do
something in a Rails 3 application:
$ rails g
This will throw the 'please type rails new foo' message rather than the
proper generator documentation message. This is because older apps don't
have bin/rails.
Therefore, we now *prefer* bin/rails, but still search for script/rails,
and exec the one we find.
This command is based on enginex gem by José Valim. It generates gem structure
and ads dummy application into test/dummy. This can be used to start developing
any kind of extension for rails 3.
