Add support to self-defined jsonp callback param, and jsonp encoding

***For context.output***

Change:
```
func (output *BeegoOutput) Jsonp(data interface{}, hasIndent bool) error
```
To:
```
func (output *BeegoOutput) Jsonp(data interface{}, hasIndent bool, coding bool) error
```

Add:
```
func (output *BeegoOutput) JsonpEx(data interface{}, hasIndent bool, coding bool, callback string) error
```

***For Controller***

Change:
```
func (c *Controller) ServeJsonp()
```
to:
```
func (c *Controller) ServeJsonp(encoding ...bool)
```

Add:
```
func (c *Controller) ServeJsonpEx(callbackParam string, encoding ...bool)
```
Other languages like PHP, have 4-space indent for json_encode.
4-space is more readable than 2-space.