Allow details to be specified in demos URLs

[RobH123]

@richmahn suggested this and I've wanted it too for a while (but don't currently know how to do it):

Instead of having to edit a page to enter the repo/file/BP details and then run the check, it would be nice if some or all of those details could be specified in the URL. Then a link to check a specific repo/file/BP could be shared.

[richmahn]

I mentioned this, but with github.io and styleguidist, I looked to see if there was a way, but I don't think so. We'd have to make this an actual app that reads the parameters, but then it is no longer a demo.