Enter key should add newline for non-Ruby files

This allows the enter key to insert a newline in a line beginning
with "#" if the document is not a Ruby source file. Returning from
the command does not fall-through to the execution of the
key-binding.

Note that inserting "\n" might cause issues with files formatted
with different EOL settings (\r\n on Windows). I have not tested
this, but I don't know any better way to handle this use case.
It seems that the plugin already uses \n elsewhere, though.