Add flag to disable file downloads

[alexfikl]

This is an improvement to #387 that tries to avoid downloading files all together (and all the additional traffic!) when the right conditions are met. For this, it adds:

• a only_data flag to get_matching_importer_or_downloader
• a only_data flag to get_matching_importer_by_name (new function in papis.utils)

so that they only call Importer.fetch_files if asked. It updates papis add and papis update to use these new functions.

It probably needs a bit more testing, but it seems pretty clean. @alejandrogallo What do you think?

[alexfikl]

Hm, this doesn't actually work as expected because a lot of importers just implement fetch in one go instead of fetch_data and fetch_files separately.

It's probably a good idea to clean that up in some future PRs so that it works nicely for all importers. Downloaders are a bit better at this because they implement get_document_url instead, so it can be skipped.

[alexfikl]

This is pretty much just a refactor at this point, since it relies on the importers to properly split fetch_data and fetch_files, so going to merge it! 🚀

[Twix53791]

I did'nt find a flag to set this option when adding entries ? I missed something? So I added the flag myself. papis add --only-data will disable file download. --force-download takes priority over --only-data.