Add Sensor, Make Sending Check Optional, Code Cleanup

[Snuffy2]

I apologize for the size of this PR. I was looking to add an option to create Sensors instead of Binary Sensors and to make sending the Check ID optional. In order to get these working, I ended up cleaning up the config flow and moving a bunch of variables into const for consistency.

Changes:

• Implement unload (Same as PR Implement unload to allow reloading integration #42)
• Send the Check first before updating the HealthChecks data, separate out the error reporting and make it more tolerant to errors (will continue to check again in 5 min) (Same as PR Send Check First & Separate Out Errors #43)
• Add option to create Binary Sensors (default), Sensors, or Both of the HealthChecks
  • The Sensor gives the state which gives a little more detail than just true/false (ex. Grace)
• Make it optional to list a Check ID. If not entered, it will not send the update ping every 5 min. Useful if you want to have entities for HealthChecks but not have a HealthCheck for your HA.
• Move a number of entity variables to const to standardize between Binary Sensor and Sensor
• Config flow optimization (Includes PR Fix potential mutable dict issue #44)
• General code cleanup

[johntdyer]

any plans to merge this ?

[ludeeus]

This needs a rebase.
This also needs to be split into sizable chunks, do 1 thing pr. PR.