PAGE_404_ROUTE          Default 404 page
callback_mapping        Create a mapping bewtween a ui element and a
                        server callack
change_page             Change the currently displayed page.
cleanup_hashpath        Formats a URL fragment into a hashpath starting
                        with "#!/"
create_router_callback
                        Internal function creating a router callback
                        function. One need to call router callback with
                        Shiny input and output in server code.
extract_link_name       Extract link name
get_page                Convenience function to retrieve just the
                        "page" part of the input. This corresponds to
                        what might be called the "path" component of a
                        URL, except that we're using URLs with hashes
                        before the path & query (e.g.:
                        http://www.example.com/#!/virtual/path?and=params)
get_query_param         Convenience function to retrieve any params
                        that were part of the requested page. The param
                        values returned come from "httr::parse_url()"
is_page                 Is page
log_msg                 Helper function to print out log messages into
                        Shiny using cat() and stderr(), as described on
                        https://shiny.rstudio.com/articles/debugging.html
make_router             Creates router. Returned callback needs to be
                        called within Shiny server code.
page404                 404 page
parse_url_path          Parse url and build GET parameters list
route                   Create single route configuration.
route_link              Route link Adds /#!/ prefix to link.
router_ui               Creates an output for router. This configures
                        client side. Call it in your UI Shiny code. In
                        this output ui is going to be rendered
                        according to current routing.
valid_path              Internal function that validates that path is
                        defined in routes.
