feat: allow configuring NewDecoder via callbacks

Since GH-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
decoder when defining it as a package global (as recommended by the
README).

```go
var decoder = NewDecoder(func(d *Decoder) {
	d.IgnoreUnknownKeys(true)
})
```

Author: lithammer

decoder.go

@@ -13,8 +13,12 @@ import (
 )
 
 // NewDecoder returns a new Decoder.
-func NewDecoder() *Decoder {
-	return &Decoder{cache: newCache()}
+func NewDecoder(options ...func(d *Decoder)) *Decoder {
+	d := &Decoder{cache: newCache()}
+	for _, opt := range options {
+		opt(d)
+	}
+	return d
 }
 
 // Decoder decodes values from a map[string][]string to a struct.

decoder_test.go

@@ -2024,3 +2024,18 @@ func TestUnmashalPointerToEmbedded(t *testing.T) {
 		t.Errorf("Expected %v errors, got %v", expected, s.Value)
 	}
 }
+
+func TestNewDecoderWithOptions(t *testing.T) {
+	defaultEncoder := NewDecoder()
+	ignoreUnknownKeys := !defaultEncoder.ignoreUnknownKeys
+
+	decoder := NewDecoder(func(d *Decoder) {
+		d.IgnoreUnknownKeys(ignoreUnknownKeys)
+	})
+
+	if decoder.ignoreUnknownKeys != ignoreUnknownKeys {
+		t.Errorf("Expected ignoreUnknownKeys to be %t, got %t",
+			ignoreUnknownKeys,
+			decoder.ignoreUnknownKeys)
+	}
+}