Add css classes to the renderer so that SAT can detect multi-columns

Summary:
Adds a `.perseus-renderer` className to the renderer's main output,
and a `.perseus-renderer-two-coumns` if there were two columns
rendered, so that the SAT content can style itself differently.

I would love if we could figure out how to do this without
these hacks, but that seems a lot more time consuming right now,
and this should get things done.

Test Plan:
In test.html, I added content with and without a `=====` column
break, and examined the perseus classes via chrome's `inspect element`.

Auditors: johnsullivan, emily