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
Once you get into manual parsing, there's always an edge case:

```js
var s = "this \
// not really a comment";
// this is the only comment
```

Do you want to yield false positives these contrived situations? I don't mind, of course.

Even though it reuses the same logic as block comments, it would take less code to only use this code for block comments.