CLI2: Added conditional expression eval

- Only non-'string' type attributes ('numeric', 'date', 'duration') support the
  evaluation of values in FILTER arguments, for example:
    due.before:now+1d
  If evaluation is supported, values need to be lexed into tokens, and if there
  are multiple tokens, parenthesize the set.

src/CLI2.h

@@ -105,6 +105,7 @@ private:
   void desugarFilterPlainArgs ();
   void insertJunctions ();
   void defaultCommand ();
+  std::vector <A2> lexExpression (const std::string&);
 
 public:
   std::multimap <std::string, std::string>           _entities;