Add white-list to WorkingCopyCheck

[evs-xsarus]

This code change makes it possible to have working copy changes and still release. For instance a CHANGELOG file that must be committed with the release, but you don't want a separate CHANGELOG commit and version commit.

Only modified files will be checked, thus those that have the 'M' in a status list.

Works with Git and HG.

Configuration in .rmp.yml:

     prerequisites:
        working-copy-check:
            allow-ignore: false
            allowed-modifications:
                - CHANGELOG.txt
                - magento/index.php