draft-6 support ("contains" keyword)

[awwright]

• [x] "contains"
• [x] draft-6 test suite
• [x] format: "idn-email"
• [ ] format: "idn-hostname"
• [x] format: "iri"
• [x] format: "iri-reference"
• [x] format: "json-pointer"
• [ ] format: "relative-json-pointer"
• [x] format: "uri-template"

See also:

"propertyNames" — implemented in 1.3.0 "exclusiveMinimum"/"exclusiveMaximum" — implemented in 1.3.0 "if"/"then"/"else" — implemented in 1.3.0

[awwright]

Implemented in 763431b48a1c6dca88160a750d0a43180fd606e7, 82e0ca06047544567698102ed6933aed2f269bd9