notmuch-restore: implement --accumulate option

Modify command line argument handling to take a --accumulate flag.
Test for extra arguments beyond the input file.

The --accumulate switch causes the union of the existing and new tags to be
applied, instead of replacing each message's tags as they are read in from the
dump file.

Based on a patch by Thomas Schwinge:

      id:"1317317857-29636-1-git-send-email-thomas@schwinge.name"