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.
Yeah, the Javascript parsing is definitely not great. I know there are Python libraries specifically designed to parse Javascript, but that's probably overkill here.

I've added some comments, but I'm not sure what code changes could clean it up. Any thoughts appreciated.