Add comment support to io methods

[bmcfee]

This pr fixes #319 by adding a comment= flag to (almost) all* data loaders. The semantics are as follows:

• By default, any line starting with '#' is skipped.
• A user can override the comment string with any regular expression. The regexp is constructed to only match at the beginning of the line. For example, calling with comment=';' uses the semicolon as a comment leader.
• Comment skipping can be disabled by specifying comment=None. This is equivalent to the current default behavior, but I don't think it should be the default going forward. It's increasingly common for annotation tools to leave metadata in the resulting data file, and we should be encouraging of that pattern.

• exceptions for wav (obviously) and pattern, the latter because the pattern format doesn't seem to have a common grammar for comments. The other formats implicitly use some flavor of CSV/TSV, which generally support comment strings.

[craffel]

Thank you sir, always a pleasure.