Parsing the whole schema can lead to errors for malformed keyword
values, eg:

```
>> JSONSchemer.schema({ 'properties' => '' })
/Users/dharsha/repos/json_schemer/lib/json_schemer/draft202012/vocab/applicator.rb:224:in `parse': undefined method `each_with_object' for an instance of String (NoMethodError)

            value.each_with_object({}) do |(property, subschema), out|
                 ^^^^^^^^^^^^^^^^^
	from /Users/dharsha/repos/json_schemer/lib/json_schemer/keyword.rb:14:in `initialize'
```

Instead, this creates a minimal parseable schema with just the `$schema`
value, if it's present and a string. That way the meta schema can be
determined as usual and then used to validate the provided schema.

Addresses: #167

This aligns the schema generation (`JSONSchemer.schema`) and schema
validation (`valid_schema?` and `validate_schema`) method interfaces, so
that they all take the same schema object types.

The default meta schemas don't inherit the provided options, because
they're initialized with their own options and cached. To override
validation-time options, they need to be passed directly to the
validation methods.

This is a little brittle because the validation-time options are listed
separately to pull them out of the options hash. I couldn't think of a
better way to split the initialization and validation options.