expand diff.groovy to support sevntu

[rnveach]

Discussed at https://github.com/sevntu-checkstyle/sevntu.checkstyle/pull/720#issuecomment-439580030 , diff.groovy doesn't support sevntu regression.

Program needs to examine given repo and understand if it is checkstyle or sevntu. Install needs to be updated to run a different command for sevntu repo. Program needs to be updated to swap sevntu versions while keeping checkstyle constant for sevntu repo.

Documentation has to be updated for new sevntu options and to describe how to run regression for sevntu.

[rnveach]

When implementing, we should try to make this feature as repo agnostic as possible. Then if we ever have a special repo like in GSoC, we can be sure we can regress on it without needing to merge it in first or do anything special.

We can just use sevntu's repo as a base which requires a property for checkstyle version, directory to run command, and any special command needed in packaging.