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.