support user switching mathjax renderer (e.g. SVG)

[cben]

used to be very broken?
currently seems mostly harmless (but not all math gets re-rendered).
At least CM widgets must be re-measured for correctness.

[cben]

HTML-CSS <-> SVG switching seems to work mostly fine (Chrome and FF on linux).
Although cursor positioning is somewhat off (should call .changed() on the CM widgets).

But MathML in FF works immediately but any further rendering (or reload) results in the math disappearing (empty widget in the DOM). This also makes it tricky to go back to a working renderer; the only remaining math you can right-click is the Mathdown logo on top :-)

[cben]

Related: in SVG and MathML modes, the math is always black, instead of adapting the surrounding color of lists, quotations etc.

For SVG, https://groups.google.com/forum/#!topic/mathjax-users/--UzmyRg0Zw suggests \color or CSS targetting svg { stroke: ... fill: ... }.