Implement comment handling for haskell-indentation-mode

Comments are a bit special when it goes with indentation. Usually we want to align comments to a similar comment in previous line, failing that we want to align to tokens in previous line. But we cannot treat comments just like token because it would break valid code, so we need to align tokens to tokens only.

For example:

```
main = do
code 1
-- comment 1
-- comment 2
code 2
```

Above we want `comment 2` to align to `comment 1`, but `code 2` to align to `code 1`.

As far as I understand this can be achieved by marking some eligible positions as 'comment only eligigle'.