Add AsciiMath support

[expikr]

This pull request adds an option to let users additionally enable the more concise AsciiMath syntax supported by MathJax.

Usage:

[output.html]
mathjax-support = true
asciimath-support = true

To type an equation, the user simply use the \`...\` delimiter.

So for example:

Output:

Input:

Euler's Formula \` e^(ix) = cosx + isinx \` defines fractional signs.

Matrix:

\`
[ [a,b],
  [c,d] ] 
\`

Fourier Transform:

\`
f(x) = int_(-infty)^(infty) F(s) e^(2 pi i x s) ds 
\`

\`
F(s) = int_(-infty)^(infty) f(x) e^(-2 pi i x s) dx
\`

Dirac Delta:

\`
delta (x) = {
    ( \infty , "," , x=0 ) ,
    ( 0      , "," , "otherwise" )
:} 
\`

[expikr]

Tangentially relevant discussion: KaTeX/KaTeX#1472 (comment)

Quick note about \`...\`: the parser actually just looks for `...` in html, the user types backslash backtick in Markdown to escape it so that it shows up as bare backtick in html output. It elegantly solves the problem of dollar delimiter collision and cumbersome backslash-bracket escaping -- taking a symbol already used for something else in Markdown, and using its escaped version to create complementary distribution for a concise symbol that you know the user will rarely type deliberately.