mathjax/mathjax-v3: MathJax v3 alpha.4

Please be advised: do not use this in production.

Please post your feedback to the GitHub Issue Tracker or the mathjax-dev mailing list.

• Internal improvements to the TeX input processing
• Add support for \mathchoice
• Added support for missing mtable attributes: align, width, columnwidths, equalrows, equalcolumns, minlabelspacing, side.
• Add support for mlabeledtr (though \tag is still not processed by the TeX input jax)
• Add support for mfenced (#86)
• Add support for mfrac attributes: linethickness, numalign, denomalign, bevelled
• Fix loading of entity definition files (#88)
• Provide method for preventing web pack errors related to asynchronous loading (#88)
• Include AsciiMath in npm distribution (#85)