Implement a simple way of migrating bibliotheca exercises

[flbulgarelli]

Many times, we need to change exercises format. This is a problem when you have a large exercises codebase, since you must migrate all them.

We need a simple way - perhaps a rake task, perhaps a rake task template, perhaps and API call - to iterate through all exercises of all guides that match a certain criteria, update each one, and save the guide.

[flbulgarelli]

@Charlyzzz I am doing some improvements on this right now