On-page partials with Interchange

So interchange currently works by requiring separate partials with content like so:

````html
<div fa-interchange>
   <div fa-source media="small" src="/path/to/template.html"></div>
</div>
````
This allows for on-page partials like so:

````html
<div fa-interchange>
   <div fa-source media="small" src="/path/to/template"></div>
   <div fa-source media="medium"> Here is my new on-page partial!</div>
</div>
````