Vendor Octicon styles

I dropped [Octicons](http://octicons.github.com/) into a project using the default [instructions](http://octicons.github.com/usage/), which resulted in Linguist deciding my project was mostly composed of CSS.

In the spirit of vendoring libraries like [Bootstrap](https://github.com/github/linguist/blob/50ab58e91f19260b11cf92b92c5b8e8c0d79f51c/lib/linguist/vendor.yml#L37) we might like to also vendor the Octicons CSS/SCSS files.

/cc @github/linguist @cameronmcefee