XBlock argument passing support

@nedbat Quick review? 

This is a port of https://github.com/edx/xblock-sdk/pull/18/files

It adds a minimal XBlock (ProfileXBlock) which is helpful for manual testing of this functionality, and when we finish it, pyfs. That block is not production-ready, but I'd also like to give a demo of this XBlock internally, and this gets it onto edge.edx.org, killing two birds with one stone. 
My general take would be to just get this working as minimally as possible, since eventually this code will die, and we'll reuse the runtime from xblock-sdk. The more functionality we put in, the more we'll want to backport.