pmi diffs: allow passing class/method names as filters

[AndyAyersMS]

Useful for zeroing in on specific methods in a large assembly...

eg I would like to be able to say

jit-diff diff --pmi --base --base_root --diff --assembly a.exe --class Test --method Foo

You can get there eventually with pmi and counting or just diff the entire assembly but there's no need to do that as we can just pass various name filters on down from jit-diffs to jit-dasm-pmi to pmi.

Likely requires fixing PMI's command parse as in #193 so we can pass more info in...