Allow plugins to add routes

[JohnAD]

From @GULPF https://github.com/dom96/jester/issues/226#issuecomment-633048738:

From what I understand there is no way for a plugin to add new routes. My use case is a CORS plugin that adds some CORS related headers to preflight requests. As a workaround I can add a options re"" route that just returns Http204 No Content, but it would be nice if the plugin would "just work" without doing this.