Option to auto-link files in BaseDir instead of importing them

This patch adds an option so files that reside in the basedir always get linked instead of imported. I know all the pros of doing imports instead of links, and I'm aware of the tablet-mode in zotfile, but I keep my pdf's on dropbox so I can mark them up from my ereader and tablet, so a one-way copy doesn't suffice. With this patch I can drop in my PDFs as normal and still keep them on dropbox.