Implement TextUnmarshaler for enum type defs

[philikon]

Currently, the way to translate an enum value from string to the corresponding numeric value seems to be something like this:

	var enumval MyProtoEnum
	err := enumval.UnmarshalJSON([]byte(`"` + strval + `"`))

This is pretty cumbersome.

golang packages like enumer provide the ability to convert enum values from string to numeric values by implementing the standard TextUnmarshaler interface. In fact, the JSON encoder/decoder will fall back to this interface if the json.Unmarshaler interface isn't implemented.

Is there any reason not to implement this interface for proto enums? I'd be happy to provide a PR.

[dsnet]

Duplicate of #866