notmuch-dump: add --format=(batch-tag|sup)

sup is the old format, and remains the default, at least until
restore is converted to parse this format.

Each line of the batch-tag format is modelled on the syntax of notmuch tag:
- "notmuch tag" is omitted from the front of the line
- The dump format only uses query strings of a single message-id.
- Each space seperated tag/message-id is 'hex-encoded' to remove
  trouble-making characters.
- It is permitted (and will be useful) for there to be no tags before
  the query.

In particular this format won't have the same problem with e.g. spaces
in message-ids or tags; they will be round-trip-able.