fix(schema): allow to pass an {Object} (options.syntax/options.stringifier)

[michael-ciniawsky]

ℹ️ Describe the big picture of your changes here to communicate to the maintainers
why we should accept this pull request. If it fixes a bug or resolves a feature
request, be sure to link to that issue.

Type

---

ℹ️ What types of changes does your code introduce?

Put an x in the boxes that apply

• Fix
• Perf
• Docs
• Test
• Chore
• Feature
• Refactor

SemVer

---

ℹ️ What changes to the current semver range does your PR introduce?

Put an x in the boxes that apply

• Bug (:label: Patch)
• Feature (:label: Minor)
• Breaking Change (:label: Major)

Issues

---

ℹ️ What issue (if any) are closed by your PR?

Replace #1 with the error number that applies

• Fixes #1

Checklist

---

ℹ️ You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. This is a reminder of what we are going to look for before merging your code.

Put an x in the boxes that apply.

• Lint and unit tests pass with my changes
• I have added tests that prove my fix is effective/works
• I have added necessary documentation (if appropriate)
• Any dependent changes are merged and published in downstream modules

[coveralls]

Coverage remained the same at 84.906% when pulling 2b37029 on fix into 30d8d51 on master.

[coveralls]

Coverage remained the same at 84.906% when pulling 1c58e12 on fix into 30d8d51 on master.