Added create-file; put major mode at top of describe-modes.

create-file could use some work, but it gets the job done for now.
Support simple formatting in mode docs.
Create parent dirs as needed in Store.write.
Expose Resource.readLines.

It's sort of ad hoc, but since we have it, we might as well let other people
use it.
Added HereDocLiteral.
Break out canAutoFill into a method.

Use it to determine if we can refill a paragraph, and only use the commenter to
refill if we're asked to refill a comment.