Support 'patch' files in exercises and solutions

[eamelink]

Currently nextExercise and pullSolution overwrite entire files. It would be cool if they would support patching an existing file with - for example - the diff between the file in the current and the previous exercise. So that students can apply one particular exercise solution to their files, without overwriting their entire file.

[eloots]

@eamelink
That's a good idea; the question is how to apply this in practice without requiring more than beginners knowledge of git. Imagine that a student changes the test code for the current exercise, solves the exercise and makes the tests pass. Now, if a patch is supplied to change a current test to the tests for the next exercise, applying that patch may result in a merge conflict; do we expect student to be able to resolve it? I don't think we can make that assumption.
Related to #180