Move mml3 filter to an mmlFilter so that forceReparse isn't needed. (mathjax/MathJax#2718)

[dpvc]

This PR move the MML3 filter from being a preFilter to being an mmlFilter. That way it will always run and you don' have to specify forceReparse: true. It also means that (in the browser) you don't have to serialize the math and then re-parse it, as is currently done, since the browser's XSLT parser can work on any DOM fragment. The node version must serialize and re-parse, however.

There is a new enableMml3 configuration option that controls whether the filter is added or not (so that if the extension is packaged with a combined configuration, it can be disabled).

This also make the mmlFilters public, as it should have been originally.

Resolves issue mathjax/MathJax#2718.

[zorkow]

lgtm.
Btw. I could not get mml3 extension to work in the lab yesterday. It could not load the .json file.

[dpvc]

I could not get mml3 extension to work in the lab yesterday. It could not load the .json file.

It should not be trying to load the .json file, as that should only occur when used in node. It works in the lab for me (using the update-lab branch).