add option to run CMDI validator on a list of files

[kreetrapper]

At the moment, you can only run the CMDI validator on a directory (structure), but for some cases it might be nice to instead provide a list of CMDI files one wants to be checked. This is especially interesting because it seems that the validator processes every zip it encounters, even when using the -F option to limit the checking to "*.cmdi". In certain setups this slows the validator down considerably.