Add invert_state config option to reverse on/off state reporting

[jheizer]

This adds the ability to invert the state of the sensors. I have one stuck in my door bell and it is annoying seeing it as "open". I know if could be changed via templates in the UI, but I think more and more people are going to be using these cheap sensors for unusual tasks to where the feature could be useful and its relatively simple.

[jheizer]

Updated of PR #40

[thelefty]

How do we add this? I can't get the config to validate. Any help would be appreciated.