continue comments on newline

This implements an option re: continuation of comments related to the discussion we had with @hadley. With the option enabled, pressing enter when within a comment continues the comment, while pressing shift + enter exits the comment.

I'm thinking a bit about whether it would be suitable to add e.g. a little tooltip / popup that explains the functionality in the settings screen as well.
You don't need this helper as fileType.isC() will pickup c++ files as well (it inspects the EditorLanguage rather than the file extension)