Implements --real-client-ip-header option. (#503)

* Implements -real-client-ip-header option.

* The -real-client-ip-header determines what HTTP header is used for
  determining the "real client IP" of the remote client.
* The -real-client-ip-header option supports the following headers:
  X-Forwarded-For X-ProxyUser-IP and X-Real-IP (default).
* Introduces new realClientIPParser interface to allow for multiple
  polymorphic classes to decide how to determine the real client IP.
* TODO: implement the more standard, but more complex `Forwarded` HTTP
  header.

* Corrected order of expected/actual in test cases

* Improved error message in getRemoteIP

* Add tests for getRemoteIP and getClientString

* Add comment explaining splitting of header

* Update documentation on -real-client-ip-header w/o -reverse-proxy

* Add PR number in changelog.

* Fix typo repeated word: "it"

Co-Authored-By: Joel Speed <Joel.speed@hotmail.co.uk>

* Update extended configuration language

* Simplify the language around dependance on -reverse-proxy

Co-Authored-By: Joel Speed <Joel.speed@hotmail.co.uk>

* Added completions

* Reorder real client IP header options

* Update CHANGELOG.md

* Apply suggestions from code review

Co-authored-by: Isabelle COWAN-BERGMAN <Izzette@users.noreply.github.com>

Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>
Co-authored-by: Henry Jenkins <henry@henryjenkins.name>