Support @mixin + link to resource

R: @addyosmani @arthurevans 

- Adds support for `@mixin`  to component docs
- Allows devs to specify an optional link as a second argument to `@extends` and `@mixin` until we support it for realz. This also helps the case where a component you're inheriting/using a mixin from is off-site.

Example:

```
@extends core-resizable https://github.com/polymer/core-resizable
@extends core-focusable
@mixins Polymer.CoreResizable https://github.com/polymer/core-resizable
@mixins Polymer.CoreFocusable https://github.com/polymer/core-focusable
```
would produce:

![screen shot 2014-12-13 at 1 25 16 pm](https://cloud.githubusercontent.com/assets/238208/5425535/eac14daa-82cf-11e4-945f-b6cbb9645ffb.png)

Here, the double mixin and extends is just an example. You'd normally not include extend if it's mixin.