option (perhaps passed to extract methods) for prettyNum-ing observation counts

[MichaelChirico]

Often in published work we need to write 12345 as 12,345.

The way I do this by hand is with prettyNum: prettyNum(12345, big.mark = ','). But as it stands I have to clean this up in the post-up when using texreg. I suppose it's possible to add a prettyNum argument to extract methods to handle this.