add github.css for html output.


git-svn-id: https://root.cern.ch/svn/root/trunk@49291 27541ba8-7e3a-0410-8455-c3a389f83636
Corrections for HTML:
- use --mathjax
- use github.css
- generate standalone code (-s)
For other styles:
- generate standalone code (-s)


git-svn-id: https://root.cern.ch/svn/root/trunk@49292 27541ba8-7e3a-0410-8455-c3a389f83636