Add new macro ref2

[byzheng]

An alternative macro ref2 is added to directly link to reference tiddler instead of the list generated by macro showrefs.

• A new document demo/ref2-macro is added
• The refnotes plugin is not updated in the document as I am not sure how to do it.

[kookma]

I doubt having several macros is a good option. May be we can add subplugin and extend the work.

By the way, please use the source folder and single tiddler to modify, this way I can follow the changes and apply it.