Add public API option 'sumStartsWithNEquals'

Docs, code, unit tests, visual tests.

Resolves #304
This usage is odd. The documentation and implementation suggest a bool here. Did you change your mind about the interface?

I could see making the interface more flexible, allowing it to take a piece of latex, and calling it something like `prepopulateSumSubscriptWith`, but on balance I don't think the extra flexibility is worth it.