Markdown: display latex output when using latex's Markdown notation.

#### Steps to reproduce the issue
After enabling markdown option in Jetpack-> settings -> writing, try below markup.
```
**This is a sample markdown page**
$f(x)$
$p(A\cup B)$
```
#### What I expected

#### What happened instead
The latex part was not rendered

We need this because, I could then write up articles in markdown entirely without worrying about latex part.

Enable Markdown in Jetpack settings and reproduce the issue with the sample markup containing $f(x)$ and $p(A\cup B)$. Trace the Markdown rendering path; done means the Markdown text remains rendered and the inline LaTeX expressions display as LaTeX output.

Written by the indexing model from the issue text.