Ensure video as main media contains canonical path for embed plugin

Annoyingly, we do not get the canonical path for the main video asset in the CAPI elements array. 
This instead needs to be jsouped out of the html body, like we have also done for inline videos. 

This is so we can then pass to the player embed plugin, for embedding goodness.

As a [wiseman once said](https://twitter.com/patrickhamann/status/540840072669851648), If in doubt:
![google chrome](https://cloud.githubusercontent.com/assets/80089/5343119/8569f560-7efb-11e4-8e88-4920fd6475cf.png)

/cc @rich-nguyen @gklopper 
needed? defaults to `None` anyway