Enable Superscripts and subscripts as defined in Pandoc Markdown extension

```
What steps will reproduce the problem?
1. In text editor, use ~word~ ( ^word^ ) to mark chars as sub or superscript.
2. Check the HTML preview

What is the expected output? What do you see instead?
If the Pandoc Extensions: superscript and subscript (see [1]) were supported,
it would be easy to markup sup and superscript which is required for most
scientific writing.
By using the HTML tags <sup>and <sub>, this would be displayed correctly in
the html preview.</sub></sup>

Please provide any additional information below.
[1] http://johnmacfarlane.net/pandoc/README.html#superscripts-and-subscripts

My patch must be reviewed carefully because i am a newbie in both js and reg ex.

```

Original issue reported on code.google.com by `benimnetz@gmail.com` on 27 May 2013 at 6:59

Attachments:
- [sub_and_superscript.patch](https://storage.googleapis.com/google-code-attachments/pagedown/issue-59/comment-0/sub_and_superscript.patch)