Fixes date parsing error in the schema.

[d0c-s4vage]

Fixes #147

Specifically

• The date field in the schema is now a string
• Unknown fields at the top level of the schema are allowed
• The styles subschema is strictly validated to help with debugging user settings

[d0c-s4vage]

@waylonwalker I think this should address the issue you described in your blog too! (https://waylonwalker.com/lookatme-slides/)