Add yaml dynamic badge

[jacobtomlinson]

The json and xml dynamic badges are awesome. It would be great to add yaml to the list of supported markups.

One initial use case I can think of would be for helm chart repos to show the latest version of a chart.

[paulmelnikow]

Great idea!

Would you be interested in making a pull request? We have a good tutorial, and since the JSON badge is already built, it's probably a matter of conditioning on json vs yaml in the URL, adding js-yaml to handle parsing, and adding some tests. If not, maybe someone else would like to pick this up!