Implement argparse CLI handling

[thrly]

This PR adds:

• Refactored interface using argparse to better manage subcommands (rather than previous complicated use of argv). This also enabled some better logic handing for the program via the subcommand handler functions.
• Clarified focus of app somewhat to viewing aggregate stats for MTG sets / search lists
• Updated CLI commands (now setlist, set, search)
• setlist now finds the 'current' release (i.e. the set most recently released) and will give stats for it with scry set latest
• Adds a stat chart for colour distribution comparisons.
• By default, search results now include all unique prints of cards, not just the unique card (i.e. it now shows variation prints in the numbers)
• Documentation improvements made both to README, and through the cli --help