Skip Duplicates #152

Feature Request

• [X ] Yes, I reviewed the contribution guidelines.

Describe your use case and the problem you are facing

I have a directory that I regularly update with images, audio, video, and documents. I want to run wp media import on this directory but have it skip any files that have already been imported. I can't think of a way to do this today other than some complicated scripting on my side that would compare file names and date modified between the two systems.

Describe the solution you'd like

I would love a flag that I could run as part of the command that could automatically do this for me. I plan on setting up a batch job to continuously import from the directory, it would be great if I could just run wp media import --skip_existing or something along those lines.