VimDiffTool is not implemented, remove arguments fir the time being

Don't register unimplemented DiffTool

Signed-off-by: Tomas Hozza <thozza@redhat.com>
Remove kwargs magic in DiffHelper and also unused code

Signed-off-by: Tomas Hozza <thozza@redhat.com>
Rename variables in Diff class to be consistent across Helpers

Signed-off-by: Tomas Hozza <thozza@redhat.com>
Rename Diff to Differ

Signed-off-by: Tomas Hozza <thozza@redhat.com>