Schema DDL: add JSON Schema Draft v7 support

[chuwy]

http://json-schema.org/draft-07/json-schema-release-notes.html#formats

In validation there's few new formats, most useful of which include: date, time, regex.

Requires corresponding ticket in Iglu client when implemented.

See also: #170

[jethron]

Would love this sooner rather than later; some super useful features in the newer drafts. Particularly "examples" and "const" are awesome, but throw errors in Igluctl and mark schemas as invalid.

[aldemirenes]

Migrated to snowplow/schema-ddl#18