Add extended_stats hook

I added an extended_stats hook. This is very useful if you want to add process-specific information into the stats. I have an example in examples/uwsgi_lossless_reload.py where I grab additional information from uWSGI's stats engine and merge it into the regular circus stats.

Since the extended stats require extra processing, I made it so you have to request extended=True in the stats command.

I updated docs and unit tests.

Hope you like it.