Add work-from-queue (-q) parameter implicitly when start(end is set
Renamed sha1_checksum to sha1, implicit updating via content

Also added fultext attribute due to repurposing of content.
Files are now written to MongoDB (GridFS)

Also, save operation for attachments now does partial updates if the
attachment is already available. Additional minor changes:
- MongoDatabase.add_attachment() renamed to save_attachment()
- File extensions now configurable via config
Command line parameters -u and -f killed

Fulltext extraction is no longer integrated, so -f/--fulltext is gone.
-u/--update is now the default behaviour and cannot be turned off.