Use a function for dataMeta

[Izabrr]

Hi SeyZ,

Thanks for your work !

In our project, we need something different for dataMeta. Indeed, we don't know meta keys before creating our serializer. That's why we want a function for the whole meta object instead of a function for each meta key.

I'm going to create a PR with the modification.

It would be great to have it in the next release ;)

Thank you,
Izabrr