Allow client to set custom duration

[siaojiecai]

@clovisnicolas Maybe we can provide a optional duration parameter in public api so that client can set vibrate duration as they ?

[clovisnicolas]

I will do it if we find a way to achieve the same result on iOS.
For now you can use patterns with no pause to have a longer vibration on both platforms

[siaojiecai]

 @clovisnicolas thanks~ actually I tried to customize the duration since our PM request a shorter vibrate duration, but as you said, it seems iOS didn't have equivalent pubic api.