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
because I use it as a callback and this:

```js
forbid(afterSingleIdxs, isNotEmpty, "Non-empty lines forbidden");
```

is more readable than this:

```js
forbid(afterSingleIdxs, function (x) { return !isEmpty(x); }, "Non-empty lines forbidden");
```