Add TextMarshaler and TextUnmarshaler options.

This feature is not enabled by default, so it won't unintentionally change behavior of existing user applications.

This was proposed by @benluddy, approved, and he already opened a PR to:

Add TextMarshaler and TextUnmarshaler options to EncOptions and DecOptions, respectively. They're essentially derivative of the existing options for BinaryMarshaler and BinaryUnmarshaler.

Enabling this improves drop-in compatibility with encoding/json, which similarly marshals non-json.Marshaler TextMarshalers to strings and unmarshalers non-json.Unmarshaler TextUnmarshalers from strings.