Make -f option action="store_true" so that it does not consume the next argument.

As it was, if you passed the -f option the following option would be consumed and stored in options.force.  If you head to [this example branch](https://github.com/transcriptic/clar/tree/optparse-error-example) and run the same clar generate command used in [libgit2](https://github.com/libgit2/libgit2/blob/development/CMakeLists.txt#L424):

    $ python generate.py -f -xonline -xstress .

You'll see that ``options.f == "-xonline"`` and that ``options.excluded == ['stress']``.