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. 
If it's not set, it will just have an empty string passed in. Not the friendliest behavior, but also not insane. We could toss in a `if($(".xblock_json_init_args").length == 0) {console.log('Warning: XBlock JavaScript expects data parameter!');}`