Make `irc_crawler.py` use argparse

It appears that a lot of modules use `getopt`. Python 2.7 introduces the
`argparse` module (and it can be installed via pip on older
distributions). This migrates `irc_crawler.py` over to `argparse` as a
demonstration. This give (IMHO) cleaner argument parsing, and really
nice (IMHO) help output.

David, feel free to reject this patch if you don't like it. No feelings
will be hurt.
Merge pull request #1 from PiPeep/argparse

Make `irc_crawler.py` use argparse