show(): added color keyword here so that the selected color can be
chosen on each invocation of askcolor().

Also fixed this class, and askcolor() so that the same Chooser
instance can be re-used instead of creating a new one on each
invocation of askcolor().

Added a module function save() which can be used to explicitly save
the option database in ~/.pynche.  This does not happen automatically
when used as a modal.