Implement generic text encoding interfaces

[chrisnovakovic]

Implement the yaml.Marshaler and yaml.Unmarshaler interfaces from gopkg.in/yaml.v2 in Version.

(I don't know whether there's any appetite for merging marshalers for data serialisation formats that aren't in the standard library, but a YAML one is something I have a need for and I figured it'd be worth opening a PR for it just in case. This only introduces a dependency on gopkg.in/yaml.v2 in the tests.)

Rather than implementing encoding/json's Marshaler and Unmarshaler interfaces in Version, implement encoding's TextMarshaler and TextUnmarshaler interfaces. These interfaces are used by encoding/json as well as multiple third-party encoding/decoding packages.

[pamburus]

It would be better to implement encoding.TextMarshaler and encoding.TextUnmarshaler interfaces.
They are in standard library and they will be used in any text marshaling, including YAML, JSON, and a lot of others.

[chrisnovakovic]

It would be better to implement encoding.TextMarshaler and encoding.TextUnmarshaler interfaces. They are in standard library and they will be used in any text marshaling, including YAML, JSON, and a lot of others.

Quite right, thanks! I'll push a new commit in a second...

[mattfarina]

Unfortunately, this is a breaking API change. Removing functions will do that. The Text ones would need to be additive.

[MarkRosemaker]

I've added them instead of replacing them in this PR and also added marshalling and unmarshalling tests for three yaml libraries:
#173

[mattfarina]

I'm closing this one in light of #173 since removing functions would change the public API which we aren't doing in a minor improvement.