customizable default action and flags for commands

Introduces the magit-command-defaults customization variable which
allows users to specify default flags to select in the pop-up command
interface and/or default actions to perform without raising the pop-up
command interface at all.

See the documentation of magit-command-defaults for full usage
information.  The default value of magit-command-defaults is taken from
two hacks previously hard-coded into the magit-key-mode-generate
function.