BREAKING CHANGE: the include type will make a copy of its options
and return a setter for the `schema`, fixing a bug in createSchema
and allowing it to take readonly options rather than mutating them.
BREAKING CHANGE: updates the underlying js-yaml from v3 to v4,
thus including all of the breaking changes from that:
https://github.com/nodeca/js-yaml/blob/master/migrate_v3_to_v4.md.
The API of this module has not changed, but some documents
may be parsed differently, so this is breaking.
BREAKING CHANGE: this allows multiple schemas to coexist, but requires
each schema to be created with a call to `createSchema`, taking a set
of options that includes the former `includeOptions` singleton.
Sean Sube