gdb/python: keyword args for Color.__init__

GDB's Python API documentation is clear:

     Functions and methods which have two or more optional arguments allow
  them to be specified using keyword syntax.

The gdb.Color.__init__ method matches this description, but doesn't
support keyword arguments.

This commit fixes this by adding keyword argument support.

There's a new test to cover this functionality.

Approved-By: Tom Tromey <tom@tromey.com>