Initial support for draft 2019-09

[zhxiaogg]

Hi,
We use json-schema in our internal project, and we want to use duration support in draft 2019-09, I'm aware the talk in this thread #329, it's indeed a big change.
After a rough reading of the draft 2019-09, I made some codes hoping it could provide any help to make the work happen. In this pr:

• managed to load schemas defined using 2019-09 draft schema, the implementation should be backward compatible with draft 7 for now.
• added integration test for draft 2019-09, and it'll allow us import more tests from https://github.com/json-schema-org/JSON-Schema-Test-Suite
• added duration format support

Feel free to merge it or close it, I can add more codes if you need, like output schema support, other things specified in the change log.

[coveralls]

Coverage decreased (-0.2%) to 91.387% when pulling 69f88ae on zhxiaogg:draft-2019-09 into a8dc686 on everit-org:master.

[erosb]

Hello @zhxiaogg , thanks a lot for your contribution. I don't merge this PR into master, but I pushed your branch into this repository, and I will use it as a base for any future draft-2019-09 implementations.
Cheers!