Allow auto-installation of plugins.

This allows Glaive to configure maktaba plugins that have been installed by
a non-maktaba plugin manager. Specifically, if the :Glaive'd plugin is on the
runtimepath but not already registered via maktaba#plugin#Install then Gliave
will do the maktaba#plugin#Install.

Note that it is not requried that maktaba#plugin#Install be called manually on
plugins. Maktaba plugins will happily install themselves at the normal load time
(after the vimrc is loaded). maktaba#plugin#Install need only be called manually
if you want to access plugin functionality within the vimrc.

This change makes it so that Glaive can do the maktaba#plugin#Install call
itself, instead of relying on an outside maktaba-compliant plugin manager.