expose accessor for value of unregistered extensions [dev]

[jhump]

This is the same as #420, but for the dev branch. This would resolve #385.

[dsnet]

Rather than adding a new function, can we change GetExtension to return the raw bytes as a []byte if the ExtensionDesc passed in is "partial"; where we define a partial ExtensionDesc as one where ExtensionDesc.ExtensionType is nil.

[dsnet]

It seems to me that this would be a forward compatible change since the current behavior is to panic since GetExtension calls decodeExtension, which calls reflect.New(reflect.TypeOf(ExtensionDesc.ExtensionType)).Elem(), which results in a panic.

[jhump]

@dsnet, done. PTAL

[dsnet]

// GetExtension retrieves a proto2 extended field from pb.
//
// If the descriptor is type complete (i.e., ExtensionDesc.ExtensionType is non-nil),
// then GetExtension parses the encoded field and returns a Go value of the specified type.
// If the field is not present, then the default value is returned (if one is specified),
// otherwise ErrMissingExtension is reported.
//
// If the descriptor is not type complete (i.e., ExtensionDesc.ExtensionType is nil),
// then GetExtension returns the raw encoded bytes of the field extension.

[jhump]

Done

[jhump]

@dsnet: I addressed your last comment. Is this ready to merge now?