Use encoding.TextMarshaler/encoding.TextUnmarshaler if present

[drshriveer]

Background:

In order to parse custom flag types it is currently necessary to implement the flags.Unmarshaler and optionally the flags.Marshaler interface. That interface is specific to this library, but many types already make use of encoding.TextMarshaler and encoding.TextUnmarshaler which is a bit more standardized. It is cumbersome, and sometimes impossible when you don't own the code, to implement both interfaces. Thus this PR allows adds the ability to marshal flags by those interfaces.

Changes:

• add interface check for encoding.TextMarshaler to convertMarshal only AFTER checking for the flags.Marshaler interface.
• add interface check for encoding.TextUnmarshaler to convertUnmarshal only AFTER checking for the flags.Unmarshaler interface.
• add round-trip test for a type that implements text unmarshallers.