Decoding into a slice of pointers doesn't work

See this test: https://github.com/zlepper/encoding-html/blob/master/decoder_test.go#L125-L153

It's probably not that common a usecase, but I have seen it happen, so it should be supported somehow. 

Internally the tests fails here: https://github.com/zlepper/encoding-html/blob/master/decoder.go#L151 Because the underlying value for the pointer is nil. 