Add config key in window-covering to offset motor response lag time
The window-covering config has keys `totalDurationOpen` and `totalDurationClose`, and accepts hex codes for open, close, and stop, allowing for percentage-open control, based on linear time interpolation.

This is great, except that I find with my curtains, there is a small delay after sending the open/close command before the motor actually starts moving, and this introduces error for percentage-open controls. When setting to the same percentage-open value, the physical result depends on whether it started fully open or fully closed.

To fix this, how about adding another key, say, `motorLagTime`, to compensate for this delay?

For example: with the following config:
```json
"totalDurationOpen" = 6000,
"totalDurationClose" = 6000,
"motorLagTime" = 400
```
Starting at the fully-closed (0%) position, setting the curtain to 50% would send the open hex code, wait 6000 / 2 + 400 = 3400ms, and then send the stop hex code. It would be the same 3400ms in the opposite direction, from fully-open (100%) to 50%. The waiting time is asymmetrical, but because of the motor delay, the curtain ends up physically at the same position.