Add NumberDecoder

When decoding JSON, elastic by default uses `DefaultDecoder` which uses
`json.Unmarshal` behind the scenes.

This commit adds a `NumberDecoder` which uses `json.NewDecoder(...)` and
`json.UseNumber()` for use cases like those described in #891.

To switch to `NumberDecoder`, do:

```
client, err := elastic.NewClient(
        elastic.SetDecoder(&elastic.NumberDecoder{}),
)
```

Of course, you can always provide your own decoder, as described in
https://github.com/olivere/elastic/wiki/Decoder.

Author: olivere

decoder.go

@@ -5,6 +5,7 @@
 package elastic
 
 import (
+	"bytes"
 	"encoding/json"
 )
 
@@ -24,3 +25,14 @@ type DefaultDecoder struct{}
 func (u *DefaultDecoder) Decode(data []byte, v interface{}) error {
 	return json.Unmarshal(data, v)
 }
+
+// NumberDecoder uses json.NewDecoder, with UseNumber() enabled, from
+// the Go standard library to decode JSON data.
+type NumberDecoder struct{}
+
+// Decode decodes with json.Unmarshal from the Go standard library.
+func (u *NumberDecoder) Decode(data []byte, v interface{}) error {
+	dec := json.NewDecoder(bytes.NewReader(data))
+	dec.UseNumber()
+	return dec.Decode(v)
+}