`mbed help` should show the help instead of fail

`mbed help` currently prints usage:

usage: mbed [-h]             ...
mbed: error: argument            : invalid choice: 'help' (choose from 'new', 'import', 'deploy', 'add', 'remove', 'publish', 'update', 'sync', 'ls', 'status', 'compile', 'test', 'export', 'target', 'toolchain', 'default', 'config')

Could we add help as a command aliased to --help?
