Initial proposal for being more CSP-friendly

This came out of some work I was doing with the requirejs-rails gem, but I hope this is more useful outside of the rails-related gem. This would give us the option to read the config value from the DOM. The requirejs-rails technique inserts a snippet of javascript with ```var require = some json object``` where this is unnecessary. Surely any application can read and set this variable themselves, but this would hopefully streamline the process.

I will gladly write tests if this is a patch you would be willing to accept.