Add support for specifying custom :meta options

[smoothcontract]

I'm currently using this with a project and it's working well for including pagination metadata in a JSONAPI using AMS and Kaminari. However I want to extend the returned metadata to include additional values other than the existing pagination keys; currently that's not possible as the entire meta: options passed to render method are overwritten.

I've opened a PR that suggests an approach - essentially this just merges rather than overwrites any existing meta: options. Let me know what you think 😁

#37