Math expressions

Usage of math expressions in Markdown text is quite common. I’ve found this PR chjj/marked#180 in marked which brings this feature. Main issue with current implementation occurs in cases like this:

$ A_{i} + B_{i} $

That inner part of $s must not be rendered as a normal piece of Markdown text. Actually, mentioned PR is not merged yet but I think supporting this expressions only needs exclusion of $s content. For example, output of this simple case:

$ A_{i} $

Is completely compatible with MathJax which renders that as its equivalent expression.

[nournia]

Using $ for inline expressions and $$ for expression blocks, is quite conventional. Because, these symbols are supported in LaTeX writing. There is a discussion about those symbols in mentioned PR.

In that PR, math expressions are treated as first class ones which use their special renderer. I think this is the correct way of math expression support, and I want it to be merged. But I think introducing a simple option for preventing lexer from analyzing inner text of $ and $$ symbols, would do.

[martinosorb]

I believe this issue is again there in mistune v2. Is there a way to avoid parsing markdown within $ and $$ equation blocks?