ignore comments in readdlm. fixes #536

This would enable `readdlm` to skip comments while parsing.
- Any line that begins with the comment char (`#` by default) would be treated as a comment.
- `#` at any other position in a line would skip the remaining of the line.
- `#` within a quoted column would have no effect.

New optional parameters to `readdlm`:
- `comment_char` : `#` by default.
- `comments` : whether to skip comments (`true` by default)