Naming convention of data attributes keys

[renz-ken]

According to the JSON recommendation (http://jsonapi.org/recommendations/#naming) the preferred naming convention is to use hyphens to separate words in keys, eg 'modified-date'. Current implementation of the library has a dependency on nilportugues/php-api-transformer which converts camelCase words to underscore-separated words, eg: 'modifiedDate' to 'modified_date', which leaves no way to conform to the recommendation. It would be great to add an option that allows to choose which transformation logic is applied to the key names.

[nilportugues]

Notice this is a recommendation, not an enforcement.

Yet I agree a parameter could be set to take this into account and allow different conventions

[nilportugues]

Code related to the issue:

Repo
https://github.com/nilportugues/php-json-api/blob/master/src/JsonApiTransformer.php#L249

Dependency
https://github.com/nilportugues/php-api-transformer/blob/master/src/Transformer/Transformer.php#L121