New feature: asciimath syntax support

[zmx0142857]

• You can enable this feature in options.html.
• Convertion from asciimath to TeX is mostly done by js/asciimath.js.
• See test/test_asciimath.markdown for examples.

Asciimath is a tiny and simple markup language for math formula markup, and it saves us time typing backslashes in TeX. Personally I have been gladly using this markup for years. Please take a look :-)