[WIP] Add config option to globally format JSONAPI type.

[beauby]

This is a response to #974. The idea is that client-side libraries have different conventions on the format of the type attribute, which this PR offers to set globally as a lambda.
I'm interested in opinions on the best way to offer a good experience with this. A few leads:

• Having a few available lambdas (like pluralize_dasherize, singularize_camelize, etc)
• Offering the possibility to chain some provided "formatters", possibly with a mix of lambdas as well (like [ :pluralize, :underscore, -> (x) { ... } ]

This is also related to #1029 (although the solution here is to globally handle the formatting, where #1029 offers to set it at the serializer level).

[beauby]

This PR actually also provides the possibility to override the type at the serializer level, simply by defining the type method on the serializer.

[bf4]

I'm thinking this should wait for deserliazation, since any deviation from snake_case would make a rails-to-rails json-api not work (and ember, fwiw, already supports this typo of mapping). esp see https://github.com/ember-data/active-model-adapter/blob/210c2302edfe2c2a3c375668411ffa81ef77dfe3/addon/active-model-adapter.js which comes with ember-rails https://github.com/emberjs/ember-rails

[bf4]

ref: rails/rails#20389