Bug Fix - #322 rc: override for shell command does not propagate

- Fixed bug #322 which failed to propagate rc overrides to shell commands.
- Context now properly records overrides to file and variables.
- The text.cpp:split (...) functions can now skip trivial split results.

src/Context.h

@@ -83,7 +83,8 @@ public:
   StringTable               stringtable;
   std::string               program;
   std::vector <std::string> args;
-  std::string               overrides;
+  std::string               file_override;
+  std::string               var_overrides;
   Cmd                       cmd;
   std::map <std::string, std::string> aliases;
   std::vector <std::string> tagAdditions;