String format validation

[bytesnz]

Updating date-time to validate only string and as per https://tools.ietf.org/html/draft-wright-json-schema-validation-01#section-8.3.1/https://tools.ietf.org/html/rfc3339#section-5.6

• [x] Move current date-time format to dateish
• [x] Add date-time format
• [x] Add date format
• [x] Add time format
• [x] Make date/time formats only validate strings
• [x] Make other formats only validate strings (as per https://tools.ietf.org/html/draft-wright-json-schema-validation-01#section-8.1)