Remove comments

Coming from Traceur, which removes comments by default, I think it makes sense to have that option built-in. This PR is an attempt to get that feature in (although by default comments are kept).

* Introduce `--remove-comments` (`-c`) option. Not sure about the naming.
* The comments are excluded from the generated code, but I'm slightly annoyed by the fact it still inserts as may newlines as there were comment lines. Any ideas on how we could remove the newlines as well.
* Tests are yet missing.

Do you like the feature at all? If so, let's discuss how to proceed.