Add YAML support via circe-yaml

[sbrunk]

I have some YAML that I need to load into a configuration (via ciris-kubernetes). I had a brief look into what's needed to get this working with circe-yaml.

Amazingly, all it took to get this working was to add the circe-yaml dependency, and replace the parser.

ciris/modules/circe/src/main/scala/ciris/circe/circe.scala

Line 12 in b34af51

import io.circe.parser.parse

- import io.circe.parser.parse
+ import io.circe.yaml.parser.parse

It just works because the parse method in circe-yaml has the same signature as the circe json parser:

def parse(yaml: String): Either[ParsingFailure, Json]

So perhaps we can extend def circeConfigDecoder to take a parser (or delegate that to a private method) and then add a yaml module that uses the circe-yaml parser instead of the json parser.

Happy to draft a PR if you're interested.

[vlovgr]

Sounds great @sbrunk! Feel free to open a draft pull request.