Add config option for detailed message for nested errors

[marmor157]

This PR adds ability to set detailed_errors feature flag in config.

Setting this enables detailed error output for errors that can contain other errors inside, e.g. AllOf, AnyOf

Example detailed message for AllOf with nested AnyOf looks like this:

Expected all of the schemata to match, but the schemata at the following indexes did not: 1.

The following errors were found:
  1: Expected any of the schemata to match but none did.
  
  The following errors were found:
    0: Expected data to be "1".
    1: Expected data to be "2".
    2: Expected data to be "3".
    3: Expected data to be "4".
    4: Expected data to be "5".
    5: Expected data to be "6".

[coveralls]

Coverage decreased (-5.3%) to 89.636% when pulling 396307d on marmor157:master into a1191fc on jonasschmidt:master.

[jonasschmidt]

I think it might be a bit "cleaner" to be able to specify an options keyword list for the error formatter, like so: ExJsonSchema.Validator.validate(schema, %{"foo" => 1}, error_formatter: {ExJsonSchema.Validator.Error.StringFormatter, nested_errors?: true}). That options list could then be passed through to ExJsonSchema.Validator.Error.StringFormatter.format/1. Doesn't seem quite right to have the error formatter be an option in the validate call but the way it operates is defined via a global config. What do you think?