Fix #402 special characters

* Fix conversion of degree smybol "°"
* Correct handling of "<" and ">" in text mode (`parseStringsAsMath` is disabled)
* Add handling of ">>", meaning "much greater" if `parseStringsAsMath` is enabled
In my book: out-commented code is a no-no and it communicates that it was once removed to fix something (or during experimenting). If you rather keep it for your own convenience, please put a comment above it to signal the intent.

But I think it's better to have two test cases, one with it and one without it. But those have to be crafted with a bit of care to showcase the handling specifically (and avoid confusing both test cases).