Add support for yaml files

[ninjasort]

Hi @woodyrew, we were chatting in Slack about this, but I think it would be awesome to have support for yaml files built in as well.

As an aside, changing the name to metalsmith-data-files might be more appropriate.

[woodyrew]

This is a great idea, I will fork this project and mark it as deprecated and superseded by metalsmith-data-files. The reason being that the API will change considerably with the re-write. The way metalsmith-json-to-files works was based upon metalsmith-contentful but this is not the most logical method IMO.

[ninjasort]

Cool, be sure to post a link here and we can close this out. Happy to help with the new repo!