Add journaling statistics?

[bsima]

I'm imagining something like this:

$ jour stats
Average words per entry: 362
Frequency of entries: 2.4 per week
Most common tags: tag1, tag2, tag3

Maybe a more table-like layout would be better. I'm not sure.

The other question is: do I even want to add a stats feature to this? Maybe that is better suited as an exercise for the user? With a sufficiently large journal, these stats calculations will take a lot of time to run. And I definitely don't want to start requiring numpy etc for this simple script.