Allow alternate sorting for genes and subjects in plotGenotype

[Jason Vander Heiden]

It'd be nice if plotGenotype() could:

1. Alternatively sort genes based on genomic position instead of family/gene name. Probably makes the most sense as some sort of ordering argument.
2. Set the subject order, probably by simply preserving input factor levels.

I can add these both, assuming no objections?

[Daniel Gadala-Maria]

For sure; makes sense to me! I had to do some roundabout reordering in a plot of my one for the latter, funnily I didn't fix the source!

[Jason Vander Heiden]

Added features in fbd8c5a.