Yaml: Support encoding.BinaryMarshaler/Unmarshaler

Is this considerably different from the support that already exists for TextMarshaler/TextUnmarshaler?

What use cases does this have?

This means supporting types that implement that interface. The support for types that implement a different interface won't make that work.

This is specifically useful for url.URL due to it's inability to implement encoding.TextUnmarshaler until Go 2:

• https://github.com/golang/go/commit/659570915481c87559f0197c9980e8cbac8e2c33
• https://github.com/golang/go/issues/10275#issuecomment-309129607

I might take a crack at this.

The same problem reported as a breakage there also looks like an issue for yaml.

It's a bit of an awkward choice to trust on the binary encoding of the type to present a nice textual representation. I wonder if it will backfire in other cases, where we'd otherwise get a pleasant representation of the fields, and now get some unreadable binary blob.

We might support BinaryMarshaler/Unmarshaler in v3 before it's final, but we'll need to first have a look to understand what the most common conventions are around it.

To echo @lrewega, this would be very useful for url.URL support.

@niemeyer v3 landed a few months ago, what are your thoughts on the issue?

ping @niemeyer . This has been a long outstanding issue, what are your thoughts on adding support for this?

Any reason _not_ to add this?