login command no longer takes email as an arg

Email is requested at a prompt. This allows us to suggest the previous
email address from the netrc as an option rather than always requiring
the user to enter it, even if they've already done so on a previous
login attempt.

Fixes #109.
better help for create
a skeleton for zsh completion with app name suggestions
cache region names for 2 weeks
don't use completion caches in a non-default cloud
organize and add all commands which only require app flag
add app flag handling for some other commands as placeholders
remove -s option for _arguments, explain other options