#18 generic variables in paperboyd config files

[wren ng thornton]

We should add just general <variable name="" value=""/> and <value-of
name="" /> tags to the paperboyd config specification so users can have
just general variables for use in other fields (e.g. in case they watch more
than one feed from the same long url, e.g. host.com/~user/foo/bar/rss/)