Update description
Add Gaku::Deck#stats returning deck statistics
Mend card moving

Cards are moved one step too far causing uncomfortable ordering. Let's
move them the correct distance instead.
Add Gaku::Options for parsing CLI options
Have Gaku:Config raise error if config invalid

Seems like a better idea to throw exception and quit early than to need
to use #valid? to know if things went well.
Show something nicer than a stack trace if config broken
Set proper error message when options not valid
Tidy Gaku::CLI a bit