The basic idea is that if we don't find a variable `foo` then we fallback to retrieving the value from the tiddler `$:/global/foo`, if it exists.
This allows us to replace the usual importvariables-based mechanism for global definitions, avoiding cluttering up the variable namespace with every macro.
In order to permit subprocedures to be overridden, we also introduce a mechanism for conditional definitions: preceding the word definition|procedure|function|widget with a + causes the definition only to occur if the specified variable doesn't already exist. In the next commit we'll apply this mechanism to the tabs macro
When occuring in a tiddler named "New Tiddler", the following construct should return ++ instead of +[[New Tiddler]]+
<$set name="myVariable" filter="[all[current]field:title[New Tiddler]]" value="">
+<<myVariable>>+
</$set>
