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.
Can we make this `--vispy-fps` for consistency with other flags (and to prevent possible collisions with user apps) ?
Might also add documentation for it in `util/_config.py`  (search for `VISPY_HELP`).