Add support for JSON Schema validation keywords

[bkjohnson]

Closes #28

This PR implements most of the "validators" vocabulary for the 2020-12 draft of JSON Schema. Specifically it allows the following properties to be used on the following types:

Numeric (Integer and Float shale types)

• multiple_of => multipleOf
• maximum => maximum
• exclusive_maximum => exclusiveMaximum
• minimum => minimum
• exclusive_minimum => exclusiveMinimum

String

• max_length => maxLength
• min_length => minLength
• pattern => pattern

The format keyword from the Format Annotation vocabulary is also allowed.

Array (Collection shale type)

• max_items => maxItems
• min_items => minItems
• unique => uniqueItems
• max_contains => maxContains
• min_contains => minContains

Object

• required => required
• max_properties => maxProperties
• min_properties => minProperties
• dependent_required => dependentRequired

---

This PR doesn't add support for the remaining keywords for "any" type:

• enum
• const

Lastly, right now these don't receive any sort of validation other than making sure that the wrong keywords don't get added to the wrong types.

[bkjohnson]

@kgiszczak I'd be happy to add an example or two to the README once you're satisfied with the changes.

[kgiszczak]

@bkjohnson it looks really great, and yes please add some examples to README.

Outstanding items to do:

1. Add examples to readme
2. Make sure 100% test coverage
3. Run it through rubocop and make sure no warnings

Thank you, outstanding job.

[kgiszczak]

Some more changes I forgot about:

4. Update changelog with your change (add it to unreleased section) e.g

   
   ## [1.1.0] - [unreleased]

Added

• [bkjohnson or your full name if you prefer] Lorem ipsum

  
  5. Squash everything to a single commit

[bkjohnson]

Test coverage shows 100% and there are no rubocop warnings for the lib/ or spec/ directories. I limited it this way because there are a number of rubocop warnings in the benchmarks/ directory and gemspec.

Also, did you know about GitHub's "Squash and Merge" feature? It's a repo setting, but it can let you squash a PR into a single commit for merging.

You'll see it in the merge button dropdown:

Here are some more details including how to change the default commit message for the squash commit.

[kgiszczak]

Awesome job, thank you.