Replace fmt.Sprint with strconv.Itoa, specify slice capacity

Uber's Go style guide outlines a slight performance benefit when using
strconv over fmt:
https://github.com/uber-go/guide/blob/dc025303c14891f54d1847396379548773e6123e/style.md#prefer-strconv-over-fmt

Also specifiying slice capacity when it is known beforehand:
https://github.com/uber-go/guide/blob/dc025303c14891f54d1847396379548773e6123e/style.md#specifying-slice-capacity

Signed-off-by: Vartan Benohanian <vartanbeno@gmail.com>

reddit/widget.go

@@ -91,6 +91,7 @@ func (l *WidgetList) UnmarshalJSON(data []byte) error {
 		return err
 	}
 
+	*l = make(WidgetList, 0, len(widgetMap))
 	for _, w := range widgetMap {
 		root := new(rootWidget)
 		err = json.Unmarshal(w, root)
@@ -301,6 +302,7 @@ func (l *WidgetLinkList) UnmarshalJSON(data []byte) error {
 		return err
 	}
 
+	*l = make(WidgetLinkList, 0, len(dataMap))
 	for _, d := range dataMap {
 		var widgetLinkDataMap map[string]json.RawMessage
 		err = json.Unmarshal(d, &widgetLinkDataMap)