Adds markdown highlight support

closes #4574

- adds highlight showdown extension with tests

## Here's what Roon does:

![Roon Highlight](http://puu.sh/dfHGo.png)

## Here's what Ghost does with this PR:

![Ghost Highlight](http://puu.sh/dfHFv.png)
derp. fixed.

On Wed, Dec 3, 2014 at 11:55 PM, Matt Enlow <notifications@github.com>
wrote:

> In core/shared/lib/showdown/extensions/ghosthighlight.js:
>
> > @@ -0,0 +1,30 @@
> > +/* jshint node:true, browser:true */
> > +
> > +// Adds footnote syntax as per RedCarpet:
>
> Footnote syntax eh?
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/TryGhost/Ghost/pull/4575/files#r21275901>.
>