A command for gathering stats about your content

I had to do a data request for a VIP client the other week, so I started on this stat command.

The key idea: generate different datum about your content. These are datum which might typically be hard to calculate on the fly.

Currently it supports requests like:
- Published post count by author for yesterday and today: `wp stat post published_posts --group-by=post_author --start-date=yesterday`
- Word count by author for the month of February in CSV: `wp stat post word_count --group-by=post_author --start-date=2013-02-01 --end-date=2013-02-28 --format=csv`

The command is designed to be easily improved to incorporate more datums or group_bys. Or comment stats too. Open to suggestions as to what should be added.

If it seems like a reasonable fit, I'll do a little more work on it, and create a man page and some tests.