Adding ability to exclude a spec from a list of specs that might include expansion.  If you add a caret '^' prefix to a target address on the command line, it will be removed from the list of targets to be evaluated.

Testing Done:
pants goal test tests/python/pants_test::

Added unit tests

Also ran these 2 commands on the command line and compared the output:

./pants goal list tests/python/pants_test/::

./pants goal list tests/python/pants_test/:: ^tests/python/pants_test/base::

Bugs closed: 315

Reviewed at https://rbcommons.com/s/twitter/r/626/