Implements --trusted-ip option (#552)

* Implements --ip-whitelist option

* Included IPWhitelist option to allow one-or-more selected CIDR ranges
  to bypass OAuth2 authentication.
* Adds IPWhitelist, a fast lookup table for multiple CIDR ranges.

* Renamed IPWhitelist ipCIDRSet

* Fixed unessesary pointer usage in ipCIDRSet

* Update CHANGELOG.md

* Update CHANGELOG.md

* Updated to not use err.Error() in printf statements

* Imrpoved language for --ip-whitelist descriptions.

* Improve IP whitelist options error messages

* Clarify options single-host normalization

* Wrote a book about ipCIDRSet

* Added comment to IsWhitelistedIP in oauthproxy.go

* Rewrite oauthproxy test case as table driven

* oops

* Support whitelisting by low-level remote address

* Added more test-cases, improved descriptions

* Move ip_cidr_set.go to pkg/ip/net_set.go

* Add more whitelist test use cases.

* Oops

* Use subtests for TestIPWhitelist

* Add minimal tests for ip.NetSet

* Use switch statment

* Renamed ip-whitelist to whitelist-ip

* Update documentation with a warning.

* Update pkg/apis/options/options.go

* Update CHANGELOG.md

Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>

* Update pkg/ip/net_set_test.go

Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>

* Update pkg/ip/net_set_test.go

Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>

* Update pkg/ip/net_set_test.go

Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>

* Apply suggestions from code review

Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>

* fix fmt

* Move ParseIPNet into abstraction

* Add warning in case of --reverse-proxy

* Update pkg/validation/options_test.go

* Rename --whitelist-ip to --trusted-ip

* Update oauthproxy.go

Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>

* fix

Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>