Support Xapian as a backend

[JoshuaPK]

In order to be able to support thousands of pages (or more) of documents, I am proposing that a paperwork-backend be created that uses Xapian instead of Whoosh. I understand that including C++ code makes deployment a bit more complicated, so I propose that we have two paperwork-backend versions, one using Whoosh and one using Xapian. More detail on Whoosh limitations here.

[jflesch]

Related to #194