Is your feature request related to a problem? Please describe.

I have cases where I'd like a collection to be made of posts in one of a number of categories.

Describe the solution you'd like

I'd like to be able to say

    filter: { field: "category", value: ["music", "dev"] }

in a filter.

Describe alternatives you've considered

Not sure how to make this happen without restructuring my categories around Decap.

Additional context

I do have a PR ready for this, and will add a doc PR shortly.