v3.4.0

Release 3.4.0 (#11)

#10 shipped the sealed Cardmarket listing methods and the opened and
sell_count fields but left the version at 3.3.0, so npm still serves a
3.3.0 that does not have them. Nothing is published until the version
moves.

Minor rather than patch: sealed gained cardmarket/iterateCardmarket/
allCardmarket and CardmarketListing gained two fields. Every addition,
nothing renamed or removed, so a 3.3.0 caller keeps working.