Add support for yaml and markdown file format

[zuzuviewer]

1. Specify the MIME type and extension for which to add support
   yaml and markdown

2. Share an example file

3. Optionally, add a reference to the specification of the file format.
   yaml markdown

[timrulebosch]

You can do this:

yamlDetector := func(raw []byte, limit uint32) bool {
	return true
}
mimetype.Lookup("text/plain").Extend(yamlDetector, "text/yaml", ".yaml")
mimetype.Lookup("text/plain").Extend(yamlDetector, "text/yaml", ".yml")

Or something more robust, such as loading the YAML file.