Filter output: --pfilter

[zx8754]

Similar to plink: --pfilter [threshold].

https://www.cog-genomics.org/plink/1.9/assoc

--pfilter causes only associations with p-values no larger than the given threshold to be reported.

[zhanxw]

This looks like a good feature. But my concern is that some test outputs two p-values, one for analytic p-value, the other for permutation p-value. Maybe I should report if any of the p-values is no larger than the threshold. @dajiangliu