Option to conditionally skip a version/transformation

[dmarkow]

This introduces a new :skip return value on transformations to allow for conditional versions (see #147).

A common use is a generic attachment field that accepts any file types. While a thumbnail version would be desirable for a PDF or JPG file, you might not want it for a ZIP file. Just uploading the original file as the "thumbnail" (like :noaction does) has several problems, my biggest two being that 1) uploading multiple versions of a large attachment wastes storage space, and 2) there's no indication when fetching URLs that it's really not a thumbnail.

With this change, when returning :skip from transform/2:

• Nothing gets uploaded for the specified version (whereas :noaction still uploads the original)
• Retrieving the url of the skipped version returns nil

I added tests, all should be passing (except for the custom S3 bucket test that was already failing before as it's trying to compare a relative path to a full URL).

If this can be merged, I'll have another quick PR for the arc_ecto repository as well so it can accommodate nil urls.

Possible changes I can make if needed:

• Make it return the default URL instead of nil (but this wouldn't provide any way to say "there is no URL")
• Add another option, maybe :default, that will return the default URL for that version as a fallback (instead of nil)

[dmarkow]

🤦‍♂️ I could have sworn I checked existing PRs first before spending time on this, but it looks like #208 is proposing something similar.

However, that PR doesn't address the returned URLs (meaning it'll return a non-existent URL when retrieving a skipped version).