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>