Add tag to imported document

[kiike]

This is a small PR that adds a feature that I use in my workflow: when I come upon an item that is interesting, I'd like to add it to papis with an automatic tag. In my workflow it's "inbox" so that i can search for all "inbox"'ed documents, which are the ones I need to review.

To achieve this, just run papis zotero serve -s tags inbox. Of course, when no tag is provided, no tags or empty tag list will be added to the papis document.

[alexfikl]

Sorry for the delay! This slipped my mind..

It all looks good to me, so in it goes :rocket:

[kiike]

Thanks!