Request: Add configuration for single logout URL

[charlespick]

Currently, clicking log out basically takes you back to the default dashboard because homeassistant logs you out then you're immediately logged back in with your SSO provider. Single Log Out is a standard implementation of SSO and should be easy to implement by modifying the logout button action when the module is configured for single log out. I intend to submit a PR for this soon.

[charlespick]

I have a somewhat working implementation at 0623b3c if anyone wants to try it out. Something weird happening with any successive login. Not sure if it's a problem with my oauth2-proxy or my module. Working on docs and will open a PR.