Since gorillaGH-59 was rejected on the basis that it wasn't useful enough to
break the public API. I took a stab at an alternate solution that
doesn't break the public API. While still allowing one to configure the
encoder and decoder when defining it as a package global (as recommended
by the README).

```go
var decoder = NewDecoder(WithIgnoreUnknownKeysDecoderOpt(true))
```