Allow both -F and -r/-R to be used in `kin`

[bpow]

I'm not sure of the rationale for preventing the use of both the freq-file option and limitation to a region or set of regions. It seems like one might want to use analysis based on a file of population-reference frequencies, but also limit to certain regions (perhaps even just for performance reasons).

Furthermore, the fact that the current code defaults to using the frq_file as also being the regions file contributes to #29 (at least with some of my files). the bcf_sr code should be reasonably efficient even without specifying regions.