collection: Support collecting posts

This adds support for moving posts to a collection. This change relies
on #13 so that we can create and delete collections during tests.

Couple notes:

• CollectPosts accepts a single struct rather than an alias and a list of structs. This makes it easy to add new optional parameters in the future.

• We're passing []*CollectPost around rather than *[]CollectPost which is done in some of the existing APIs. The reasoning for this is:
  • Idiomatically, slices are passed by-value ([]foo) rather than by-pointer (*[]foo) because slices are already reference types. Plus they're really cheap to copy since they're just a triple: pointer, length, and capacity ([related blog post][1]). We need pointers to slices only when we're trying to modify the original slice reference and don't have the ability to return a slice, like with json.Unmarshal (see also [this post][2]).
  • If we're trying to reduce copying, []*foo is better than []foo because otherwise foo will be copied when manipulating or accessing entries in the slice (like ranging over it).
    
    Existing APIs were left as-is to avoid breaking them. I can switch to *[]CollectPost if you'd prefer that but []*CollectPost is more idiomatic.

[1]: https://blog.golang.org/go-slices-usage-and-internals#TOC_4.
[2]: https://blog.golang.org/slices#TOC_5.

Resolves #4.