Pretex

- Improved comment handling in tricky auto_align situations
- Added substack and paren trafos
- Extended arrow trafo with xrightarrow syntax
- added string input mode

- just fixing pypi package imports

- Improved sub_superscript: Can have + and - inside
- Added optional aggressive sub_superscript mode with mandatory trailing whitespace
- Changed "skip" command line parameter to more general "set" parameter to allow more nuanced settings
- auto_align improved and activated by default
- Added experimental html output