Commands - shell

- Migrated handleShell to CmdShell.
- Note there is a segfault bug that will probably go away by itself when
  Context::parse is obsoleted.

src/main.h

@@ -79,7 +79,6 @@ void handleUndo ();
 void handleMerge (std::string&);
 void handlePush (std::string&);
 void handlePull (std::string&);
-void handleShell ();
 int deltaAppend (Task&);
 int deltaPrepend (Task&);
 int deltaDescription (Task&);