Fix multiline strings.

This enables multiline strings. Credit goes to [some guy named bigeasy](https://gist.github.com/bigeasy/3902914). Please merge!

NOTE: [javascript-libraries-syntax](https://github.com/othree/javascript-libraries-syntax.vim) breaks this, because they use the same rules as you for strings. If you want to try it out, uninstall `javascript-libraries-syntax`. Another pull request against their master branch is forthcoming :)