Don't require a name in variable-expanding plugins.

  We've changed the registration of non-formatter plugins
 to just require the class name, like so:

    Templer::Plugin::Factory->new()->register_plugin( "My::Class" );

  That replaces the old method of:

    ->register_plugin( "name", "My::Class" );

  Since all plugins are called, in order, for each page a name
 is redundent.  (We keep the name for the formatter plugins as they
 are looked up by name.)