Add configurable mute options.

[doouding]

In some case, we want the video to be mute during the playing time. And we don't want user to adjust volume. In other words, there is no volume control on player ui. I check out the whole api document. It seems that dplay doesn't have any related feature. But there is a property in native video api called muted which indicate whether the video is muted or not. Can you add some configurable feature relative to this.

[zhiga90]

same issue