recursively expands ${VARIABLES} within the value (w/o refactor)

My editor ate trailing spaces, but it should be clean enough.  Fixes #39.
I carry it along in my toolchain since adding it to YUI a while back:

https://github.com/apm/yui3/blob/8dabf64ce9f41685925f1076e4bb255fdb331230/src/yui/js/yui-lang.js

It would be nice to document it, but it isn't likely to need to be changed.