Fix for Trusted Proxies and use ClientID()

[pewter77]

Not sure if you accept pull requests so here goes? This is related to #10

Sets this to work correctly on ClientID() which defaults to X-Forwarded-For behind a trusted proxy. Since this should be behind traefik and traefik decides which proxies it trusts before we can keep this set as 0.0.0.0/0 but there is an option to set this with the TRUSTED_PROXIES environmental variable.

Note that this might not be as performant as setting SetTrustedProxy(nil) and using ClientID() if you don't have proxies before Traefik. I wanted to add in some logic to set SetTrustedProxy(nil) instead if TRUSTED_PROXY wasn't set at all but my experience with go isn't that great and honestly not sure it even matters in most use cases for this bouncer but it is a potential for improvement if you want the previous functionality to stand.

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[aleksandarmomic]

+1 Please review this 🙏