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
x add yard docs

- refactor Help::Cmd and Table so that everything goes into one big table
- Ui classes really need to write error messages to stderr, not stdout
- make sure option aliases define accessors and predicates (see examples/readme/alias)