New rule: lines-around-comment

(Dis)allow empty lines around comments. This patch only implements the
lines /after/ a comment line or block. If it is accepted I will adapt it
to consider preceding lines.

The implementation is fully text-based. There is no reliable access to
comments through the official AST API, so I had to resort to manual
parsing. Luckily, JS has some quirks that make it doable.

See https://github.com/eslint/eslint/issues/1344
It's a stylistic preference. I'd encourage you to do a little research on style guides and come up with something. Part of why we require good documentation with rules is so I'm not stuck needing to write ask the documentation myself. Also, if you can't explain why someone would want to use a rule you wrote, that's probably not a good sign. :)