gh_install_packages     Install Packages from GitHub
gh_list_packages        Get Information of Packages on GitHub
gh_search_packages      Search Packages from Titles by Regular
                        Expressions
gh_show_source          Find source code for functions in packages on
                        GitHub
gh_suggest              Suggest Github Repository from a Incomplete
                        Name
gh_suggest_username     Suggest Github Username from a Faint Memory
gh_update_package_list
                        Update the List of Packages on GitHub.
recommend_dependencies
                        The default "dependencies" is NA that means
                        c("Depends", "Imports", "LinkingTo"). If
                        "build_vignettes" is TRUE, the install needs
                        "Suggests" dependency in many cases. So we
                        recommend in such case to set "dependencies" to
                        TRUE that means c("Depends", "Imports",
                        "LinkingTo", "Suggests").
remove_conflict_repos   We want to detect the two conflict cases as
                        fllows: 1. The package is already installed
                        from some repository like CRAN that is not
                        GitHub. 2. The package is already installed
                        from GtiHub but the username differs. In the
                        above cases, we ask whether to overwrite it and
                        remove from "repo" if the answer is no.
select_repository       Suggest candidates from "package_name" and make
                        user selected one of them.
separate_into_package_and_reference
                        The "repo" argument allows to contain "ref" as
                        "package_name@ref", "package_name#pull" or
                        "package_name[branch]". The function detects
                        that "repo" contains "ref" and separates into
                        pure repo and ref. If "repo" contains "ref" and
                        "ref" argument is specified, the values in
                        "repo" take precedence.
