doc: add execute command
doc: reorder commands

The commands are now ordered roughly in the order we expect users to
use them, i.e., with the important 'zpm new' first and the less
essential 'zpm version' last.
commands: refactor cmdline args for login

This gets rid of the duplication introduced in afe94a0716cd. We can
reuse this pattern for future groups of command line arguments.
commands: extract reusable pattern from login_args

We can now easily group other command line arguments in the future
without having to duplicate the logic.
Merge pull request #117 from mgeisler/execute

Improve handling of command line arg groups