Annotation plugins

As summaries here [#203](https://github.com/SassDoc/sassdoc/issues/203#issuecomment-56295987). 
I implemented the ability to pass in annotation plugins from a theme by exposing an array of annotations like `theme.annotations`. A annotation will have the same strucutre as found in `/src/annotation/annotations` with an additional `name` key. 

So an annotation in a theme will look like this:

```js
theme.annotations = [{
   name : 'NameOfAnnotation',
   parse : function(){},  // Parse this annotation
   resolve : function(){}, // Post process this annotation 
   alias : ['AliasOfAnnotation']
}]
```