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 actually left it on purpose to make it easier to re-enable, if somebody wants to try the effect of `parseStringsAsMath`. I'll remove it nevertheless.