Use new None output for mml-only output

[dpvc]

Resolves issue #378

[pkra]

LGTM

[dpvc]

I'm thinking about adding a do-nothing output renderer and using that for this case instead. Another use case would be for input syntax checking, where the output isn't really needed.

[dpvc]

I've added a None output jax that does nothing and can be used in the case where just MathML output is needed (since the toMathML() function is used for that, not the output of the NativeMML output jax), or when just syntax checking of the input is needed.