alias "rails plugin" to "rails plugin new"

origin: #10229.

As we only have "rails plugin new" left it is convenient to alias it to "rails plugin". I tripped a few times myself only calling "rails plugin" and if "new" is the only argument left we can safely do the alias.

I think it is handy but I don't have a strong opinion about this one. If you feel like this is a bad change, just close.