Update default.hbs with LaTeX math support and Syntax Highlighting

LaTeX Example: 

Input this directly in markdown:$x\_{1,2}=\dfrac{-b\pm \sqrt{b^2-4ac}{2a}$
Note that '_***' will be interpreted by markdown engine as emphasis, '\_***' is a work around.

Syntax Highlighting: 
'''
int main()
{
  return 0;
}
'''
LaTeX support using MathJax

LaTeX Example: 

Input this directly in markdown:$x\_{1,2}=\dfrac{-b\pm \sqrt{b^2-4ac}{2a}$
Note that '_***' will be interpreted by markdown engine as emphasis, '\_***' is a work around.
Merge pull request #3 from yzmtf2008/master

LaTeX equation support using mathjex