add ScreenshotScale that allows specifying page scale factor

[ZekeLu]

After the change, there will be four functions for taking screenshot:

func Screenshot(sel interface{}, picbuf *[]byte, opts ...QueryOption) QueryAction
func ScreenshotScale(sel interface{}, scale float64, picbuf *[]byte, opts ...QueryOption) QueryAction
func CaptureScreenshot(res *[]byte) Action
func FullScreenshot(res *[]byte, quality int) EmulateAction

It's a little mess, I hope that we can come up with a better API to unify them all.

Fixes #1191.

[mrueg]

Thanks a lot for creating this change! It will definitely be helpful for https://github.com/kovetskiy/mark :)