Fisher's Exact Test

See: 
https://github.com/brentp/bedtools2/compare/stats?name=stats&short_path=8105a18#diff-8105a189f30f5155ac1b1bfbe2ee2457

This is currently based off of `jaccard`. In the future, I would like to port this to have all the options of `intersect` so that we could in/exclude regions, etc. For now, this can be quite useful as-is.

I pulled in @lh3 's implementation of kfunc.c, and put it in the src/fisher/ directory.


Usage is

```Shell

   bedtools fisher -a a.bed -b b.bed -g t.genome

```