Exercise demo

Quick and dirty demonstration of automatically generated 'fill in the blank' exercises.

Of course a little too hardcoded right now, eventually a user should be able to define which exercises he wants to generate, maybe even give several rules through which we can cycle. It nicely shows how plugins can play together to create something better.

The most interesting point in this Pull Request: To get this demo up and running took only two hours! 

Also includes a new configuration option for plugins, which can declare if they don't want to have a view. Just pass in no template or set `noView: true` in a plugin's configuration.