Add mathml support

[Quafadas]

This is a sub issue of raquo/Laminar#172.

The goal, is to deal with the remaining parts of the scala-js-dom roadmap for that issue. There are four issues to work through in the four headers below.

Please do all on the mathMl_2 branch

Add MathML tags and attrs to SDT (raquo#105)

Summarise what remains to be done. Note that mathml tags are reasonably well agreed. You may review the implementation here;
shared\src\main\scala\com\raquo\domtypes\defs\tags\MathMlTagDefs.scala

but attributes are not yet started.
shared\src\main\scala\com\raquo\domtypes\defs\attrs\MathMLDefs.scala

Add mathml example to SDT tests

Review existing tests and add a mathml example in a similar style to the existing tests in this file.

js\src\test\scala\com\thirdparty\CompileSpec.scala

in particular including any attriubutes.

Fix xmlns prop – I think it should be an attribute? But not just an HtmlAttr – it needs to work for all elements.