Support multiple documents in single file

[braydonk]

The default Unmarshal behaviour in yaml.v3 will only read and output the first document in a multi-document yaml file. I was unaware that this was possible, let alone common in Kubernetes so I did not test it initially.

The way to make yaml.v3 parse these properly is to to make a Decoder for the whole document, and decode them one at a time in a loop. This behaviour might diverge heavily from the basic formatter, so it might need to be an entirely new one, but I will see if I can make it work by default.

[braydonk]

Since this looks like it will take more thought than the other v0.2.0 things, I'm going to target this for v0.3.0.

[braydonk]

Actually now that I've looked into this it's pretty doable. Gonna put it in v0.2.0