Support new option "github-user" (#421)

* feat(github): support new option "github-user"

* feat(github): rename github-user to github-users

* feat(github): update docs for github-users option

* feat(github): remove unneeded code

* feat(github): remove logging

* feat(github-user): use github-user as flagset options

* feat(github-user): remove optionns.go

* feat(github-user): add github-user flagset

* feat(github): improve readability in the docs

* feat(github-user): refactored SetUsers method

* Update flag description

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

docs/2_auth.md

@@ -103,6 +103,8 @@ Note: When using the Azure Auth provider with nginx and the cookie session store
 
 The GitHub auth provider supports two additional ways to restrict authentication to either organization and optional team level access, or to collaborators of a repository. Restricting by these options is normally accompanied with `--email-domain=*`
 
+NOTE: When `--github-user` is set, the specified users are allowed to login even if they do not belong to the specified org and team or collaborators.
+
 To restrict by organization only, include the following flag:
 
     -github-org="": restrict logins to members of this organisation
@@ -119,6 +121,10 @@ If you'd like to allow access to users with **read only** access to a **public**
 
     -github-token="": the token to use when verifying repository collaborators
 
+To allow a user to login with their username even if they do not belong to the specified org and team or collaborators, separated by a comma
+
+    -github-user="": allow logins by username, separated by a comma
+
 If you are using GitHub enterprise, make sure you set the following to the appropriate url:
 
     -login-url="http(s)://<enterprise github host>/login/oauth/authorize"

docs/configuration/configuration.md

@@ -56,6 +56,7 @@ An example [oauth2-proxy.cfg]({{ site.gitweb }}/contrib/oauth2-proxy.cfg.example
 | `--github-team` | string | restrict logins to members of any of these teams (slug), separated by a comma | |
 | `--github-repo` | string | restrict logins to collaborators of this repository formatted as `orgname/repo` | |
 | `--github-token` | string | the token to use when verifying repository collaborators (must have push access to the repository) | |
+| `--github-user` | string \| list | To allow users to login by username even if they do not belong to the specified org and team or collaborators | |
 | `--gitlab-group` | string | restrict logins to members of any of these groups (slug), separated by a comma | |
 | `--google-admin-email` | string | the google admin to impersonate for api calls | |
 | `--google-group` | string | restrict logins to members of this google group (may be given multiple times). | |