Support yaml separators

[f-f]

Use-case: I have a List {...}, and I'd like to map over that list and output several yaml objects (to the same file with separators, or to different files, not really relevant).
There is some ways to solve this issue, e.g.:

• output the values to bash var separated by newlines, run a for loop to run dhall-to-yaml either piping to the same file after piping there a separator, or to different files
• write a haskell program that reads the list, converts each value to yaml, and outputs to file(s)

It would be nice if dhall-to-yaml had a flag (--separators?) such that if the top object is a List, every element would be converted to yaml, and there would be a separator in between.

Something like:

$ dhall-to-yaml --separators <<< "[1, 2, 3]"
1
---
2
---
3

[f-f]

Relevant issue in Data.Yaml

[f-f]

Turns out it's easier to implement than I though, so I'm putting together a PR for this