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
The error indeed can be triggered by the (strange) string `x^°` (not by `x^\circ` since nothing is replaced). I will change the code accordingly.