Support multiple possible values in a collection filter

**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

```yaml
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.