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.
+1 for consistency with other options. Was the motivation for using `--vispy-*` to ensure that the `argv` were designated specifically for our code?