Asciimathml meta issue for 2.x

This issue is for keeping track of asciimathml changes which should be pulled back into MathJax for the next release.

• mathjax/asciimathml#14 -- switching phi and varphi, i.e., changing the defaults we set in #353 (closed)
• mathjax/asciimathml#19 -- leading space in quotes problem #883 (closed)
• #851 (closed)
• power of f and g #431 (closed)
• underscore #741 (closed)