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
I have no idea why anybody would want to enforce or disallow this. I'm not making this for myself, but as a fix to <https://github.com/eslint/eslint/issues/1344>, which was marked as "accepted". For a motivation, better ask the person that marked it as "accepted".

Or maybe you have an idea? I can't think of anything.