Command line interface should use argparse

[johnwmillr]

https://github.com/johnwmillr/LyricsGenius/blob/6a91cd2ac1ef100cbd5cab5f55112963b7b91ca0/lyricsgenius/__main__.py#L17

The current method for accepting inputs from the command line uses ad-hoc string parsing. The proper way to parse command line inputs is argparse.

Switching to argparse should be fairly straightforward, maybe a good first issue.