Add 'meta' property on linked data object

[eduardmartinez]

Sometimes, it's necessary to send additional data on a relationship (a field's name, etc...) in order to be processed by JSON API backend. The spec says META property is optional.

[eduardmartinez]

@jakubrohleder Sorry for bothering you, but I'd like to know if you can check it because I'm using this package on my project and it's very important. Thank you.