Strict mode flag

[alecgibson]

We recently merged some stricter type checking in #40

This works well for "greenfield" docs, but anyone already using json0 may experience breakages when rebuilding snapshots from old ops that were submitted before this stricter checking was added (this happens particularly in ShareDB's fetchSnapshot() flow for rebuilding historic snapshots).

We should probably hide these newer features behind an opt-in strict mode flag before deploying a new release. We could then also merge #23 and hide that feature behind the same flag.