Refactor opening/closing logic; persist state

[dxdc]

Made a number of changes and improvements:

• Persist cached blinds position across Homebridge restarts
• Simplified/refactored logic for calculating current blind position and whether or not to send Stop command
• Also send 'Opening' or 'Closing' state
• Added response_lag parameter to improve % calculation
• Tweaked config settings, made certain settings optional and added default values
• Minor improvements to code formatting/structure

[dxdc]

@zwerch I made some additional edits here which I'm very pleased with.

What I realized is that the blind% was not being accounted for properly because of response delays (in my case) between the server command and the blind movement. In the case where an additional stop request is needed, that is actually another delay and the target% will overshoot.

To deal with this, I've made the following additional changes:

• Initial wait time of response lag is applied before starting interval count
• Stop command is sent before final position is reached

This is not going to be perfect (e.g., when changing from 41% to 45%) but it should be fairly accurate for major changes.

I also discovered that there is no need to set PositionState; HomeKit handles this for us using CurrentPosition and TargetPosition.

Would appreciate your review! And, thanks for creating this plugin.