MathJax: Allow user to configure MathJax

The user can provide his own mathJax.config object.

Supersedes: https://gerrit.wikimedia.org/r/61182
PS2: Add relevant info for this feature to README

Bug: 35478
Change-Id: I97800317366aafcaad87ecf6644e9417c698c5da
Merge "MathJax: Allow user to configure MathJax"