Add asciimath support

[expikr]

This pull request adds an option to let users enable the less verbose AsciiMath syntax supported by MathJax.

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