add a global --color option

If you want to turn off colors for commands, you have to manually adjust
the git config settings (in various locations).  If you're writing scripts
though, you often don't want to modify those locations.  Add a commandline
option to explicitly control things.

The default behavior is unchanged -- we still scan the config files.

Change-Id: I54a3fd8e1918bac180aadd7c7d3004f069b02522