Add command-line option --fps to display FPS.

Allow to do `python example.py --fps` to display the FPS.

The argument name, the output (display the FPS in the console or directly in the screen?), and the implementation are all subject to discussion! But this feature would be really useful.
That's right--we parse command line options directly from sys.argv, so if a use writes an app that uses `--fps`, they might be surprised to find that vispy is using the same argument.