Configure schema json path

[piotr-galas]

Followed this issue #23 I want to ask you about softer version of 'magic'

Just allow user to configure schema path in initializers, and then follow convention:
path_to_shemas/model_name/field_name.json_schema

And In model use validator in this case:

validates :profile, presence: true, json: { schema: profile.json_schema }

I can prepare PR.