Set default page limit on the resource

[wgator]

We're using this option to control the size of the page when its not explicitly passed through request params, setting it as an additional resource option:

jsonApi.define({
  namespace: 'json:api',
  resource: 'media',
  pageDefaults: {
    limit: 100
  },
  ...
})

This of course could be achieved on the handlers, but I think makes sense let this be a resource option since a private default value is already being applied by the pagination.validatePaginationParams method.

The implementation is pretty straightforward, and I would be glad to match a pull request to add this option.

[paparomeo]

It looks like a reasonable extension of the resource definition @wgator and I'd be glad to merge a PR for this functionality if you'd care to submit one.