CLI 'pixel' command

[WillForan]

Minimal implementation of a clean-pixels command option for the CLI (#284).

• deid/dicom/pixels/clean.py:_get_clean_name: refactor to allow no clean- prefix and no extension change for predicable dicom_save (new directory only, not new names). Not sure if that's wise -- just how I'd originally written the test and another user might expect output data.
• deid/main/pixels.py: new file runs DicomCleaner detect, clean, save_dicom on input directory

I think it'd be useful to do both header and pixel scrubbing in one interface so everything described in a deid profile/config file can be applied at once. I'm not sure yet what the best way to go about that is (run identifiers into a new temp dir, rerun clean on that and save output to final location?)