Enhancement - gc/shadow

- shadow file update is now triggered by the type of command, not
  explicitly by name.
- Context::inShadow now keeps track of whether it is running a shadow
  file update to prevent recursion.

src/Context.h

@@ -78,6 +78,7 @@ public:
 private:
   std::vector <std::string> messages;
   std::vector <std::string> footnotes;
+  bool                      inShadow;
 };
 
 #endif