Add markup support

[bryanjhv]

Currently only Markdown, Textile and AsciiDoc are supported as formats. But GitHub has support for many others, which I'd like to see implemented here, in order to have this project complete.

Steps:

1. Choose a markup to implement. (only one)
2. Find a NPM package for X to HTML conversion.
3. Add that package as dependency on package.json.
4. Edit lib/convert.js and call register for your solution.
5. Submit a PR telling which markup format have you added.

Requirements:

• That package should not call external commands.
• Preference is given for markups supported in given link.
• Try to conform code style of entire project (spaces, others).