Add the ability to control what [part of] the solution is added to a studentified repo #170
Description
As a course maintainer, I want to have control over which parts of the solution gets added to the studentfied repository.
The default behaviour is to add the complete solution to the studentified repo. Hence, a student can pull the full solution using the cmtc pull-solution or part of the solution by using the cmtc pull-template command.
In some use cases, that undesirable. For example, a course may want to grade code written by course participants. It's obvious that in that case, one doesn't want to give the student the ability to get pull the solution.
Controlling what gets added would be done via configuration. This would be done on a per exercise basis by adding a .cmt-config configuration file in the root folder of the exercise (for example, code/exercise_005_do_some_exercise. The configuration value (key name TBD) would be a list of files that should be added to the studentified repo. If the setting is an empty list, no part of the solution is added.