Expressions Refactor - Clarified expansion rules

- Arg now has a _value member that reflects the value in play, rather
  than the input _raw value.  Only _value is used in eval.
- DOM expansion capabilities are now controlled by rc.dom.

src/Arg.h

@@ -44,6 +44,7 @@ public:
   bool operator== (const Arg&) const;
 
 public:
+  std::string _value;    // Interpreted value
   std::string _raw;      // Raw input token, never modified
   std::string _type;     // Data type
   std::string _category; // Categorized argument