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.
BREAKING CHANGE: rather than use the `fs` functions (`existsSync`,
`readSync`, etc) to include files, this uses fields of the `includeSchema`.
To maintain the previous functionality, fields should be set as follows:
- `exists = existsSync`
- `read = readSync`
- `resolve = realpathSync`
ssube