Update config.json for frac and minus

Hi Daniel
Not sure entirely about these. though I think they provide minor improvements. So, merge, or not, as you see fit ;-)
Uses longer dash − rather than minus - for better visibility in editor interface
uses \frac rather than \over for fractions
Update configuration.html

Appreciate this is a work in progress, but I have been testing this with some more complex TeX and mhchem expressions.

So, have added handling of a few more MathML tags: msub, munder, mover and munderover.

However, still does not yet deal with some more complex patterns of MathML tag attributes such as:

<mspace width="-.1667em"></mspace>
<mstyle mathsize="0.85em"></mstyle>
<mpadded depth="0" height="0"></mpadded>
<mstyle displaystyle="false" scriptlevel="0">

etc
Merge pull request #2 from geoffrowland/master

Thanks of the attention to this Geoff.  After the configuration script is perfected, we should be able to merge it into the editor so that typing in TeX expressions to an input box in the editor should also, be possible. Some functions of MathJax are not as well documented as others.  In part because they may not expect them to be used that often.