The `HyperFieldDecoder` [needs the hypermedia package's Hyperlink type](https://github.com/lostisland/go-sawyer/blob/6570b8bacd2bc3d74897b2be63e88fe419276104/hypermedia/hypermedia.go#L113-L115).  

We could pass an optional type to `HyperFieldDecoder()`.  

    HyperFieldDecoder(root, reflect.TypeOf(Hyperlink("blah")))