Add orange-cat, a Markdown previewer.

Hi,

I've just released a Markdown previewer written in Go, named `orange-cat`. It's a quite simple tool, which can hopefully be used in several editor plugins.

I added it under the `blackfriday` item, cause it uses the module for Markdown processing.

It's still `v0.1.0`, but I've added test cases covering every module and set continuous integration with Travis CI.

I hope many people like and enjoy it :)

Cheers,