at command level, attach the `shorthand` Hash to the completion
function. The completion command will then try to complete shorthands
and longer flag based on user input.
completing after just one `-` will complete the list of short flags.
completing after just two `--` will complete the list of longer flags.
- results of the first lookup is cached to
~/.bower/completion/install.json
- consider caching in ~/.bower/cache instead, so that cache-clean
command also removes completion cache.
