Add Treasury Inspector General for Tax Administration

It's a fair amount of code since there are a few different page formats, but it's actually pretty clean.

I'm not sure what the `agency_name` should be here.
This looks to me like a complicated, limited, hard to read makeshift CSV parser. It seems if the input format is legitimately CSV it would be better to use a proper CSV library. Of course, that does make me think about if there is any way to limit dependencies on a per-scraper basis, so that those who don't intend to run this scraper don't have to install the proposed dependency. Maybe there is some utility to this approach, but it seems brittle (and hard to read).