x add args descriptions
x add Cmd#examples
x add min to opts
x error message for required option (see readme/required)
x document arg splats, types
x make sure runners raise proper exceptions if a command was not found
x register runners, accept any object as a runner, too
x readme passing a block to opt
x complete readme (e.g. description, summary, examples)
x add toc

- add yard docs
- refactor Help::Cmd and Table so that everything goes into one big table