Feature request: add clean command

[0bcMoon]

add a clean command to remove empty file and testcases

[xeluxee]

This looks like a unusual use-case.
Why would you have empty testcases?

[0bcMoon]

After the contest, not everyone is able to solve all the problems, so they might want to clean up the empty source files and all the test cases

[xeluxee]

they might want to clean up the empty source files and all the test cases

CompetiTest supports templates for received problems and contests.
If a file isn't edited during a contest its may not be empty due to templates.