Allow multiline strings

This is a first pass at adding support for multiline strings, something I definitely need for browsing the HaskellNet package.

I assume you may want to handle the variable g:haskell_multiline_strings slightly differently. And then, of course, there's the documentation and testing situation. Docs I can handle - not sure how you'd want to do testing.

Let me know about the variable and testing and I'll add commits as necessary. (I'll add a doc patch in the next couple minutes.)