Add support for configurable and extendable validators

[arentrue]

This is one of the two PRs which are the result of PR #53 split.

[lazedo]

@arentrue i think the initial external validators may not be fully explained.

we have draft3/draft4 schemas that validate data, and.., for business extensions (validate a customer in a database, ....) we use the extension validator by using the callback. we look for annotated schema elements and perform whatever we want as an extra validation.
we still want to use v3/v4 validators for types, ranges, whatever. we are only interested in providing an extra layer of validation not tied to jesse core library not replacing by a custom validator.

[arentrue]

@lazedo thanks for the explanation. Yes, that is also clear from the code and description in your initial PR.

The reason I still propose this PR is because it provides a superset of the functionality you've implementated. In addition to the extra validation of annotated schema elements on top of draft4/draft3 it allows easily override original validation logic of a base validator (e.g. draft3 or draft4) for particular schema element. It also allows to create a new custom validator modules altogether.

In my opinion, it also corrects the initial validation API in jesse, moving the looping over schema into the common jesse_schema_validator module and leaving validator modules only pure validation functionality with a clean interface for that.

[lazedo]

@arentrue thanks for the reply, but how would you validate a draft4+(custom for a single field)?

[arentrue]

@lazedo answering your question in the main thread, here is an example in tests.
The customDef schema parameter is validated on top of draft4 schema, with a custom logic.

[handrews]

Note that we're now working on standardizing how schemas can be extended. json-schema-org/json-schema-spec#602 is an example. It's working through some particularly complex cases involving newly proposed keywords, and it's a lot of ideas that are new and somewhat tentative. But anyone who is interested in making extensible implementations may want to look at that issue and json-schema-org/json-schema-spec#561