Adds support for passing JSON parameters to initialize XBlock

This allows this XBlock initialization to pass a JSON argument from Python to JavaScript. This eliminates lots of template hacks getting data into XBlocks. 

There is a matching commit in the XBlock repo. 

@cpennington @rocha 
I'd like to change the code later regardless. It should be: 
  if frag.json_init_args is not None:

This is more concise and readable. I just don't want fine coupling between versions of XBlock and xblock-sdk if it's this trivial to avoid. This gives backwards compatibility if people git pull on one or the other (and not both)