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
I was quoting pure LaTeX strings (because that's where the error would arise) :smile: 
And I agree completely that it is quite unlikely that somebody would encounter it, but rather safe than sorry now this piece of code is still fresh in our minds.