Allow the use of global config file

If not found elsewhere, look for the config file in /etc/todo/config.
`make install` installs a global config file, setting TODO_DIR=~/.todo.
todo.sh creates TODO_DIR if it doesn't exist.

Makefile

@@ -33,9 +33,9 @@ clean:
 install:
 	install --mode=755 todo.sh $(INSTALL_DIR)
 	install --mode=644 todo_completion /etc/bash_completion.d/todo
-	mkdir -p ~/.todo
-	cp -n todo.cfg ~/.todo/config 
-
+	mkdir -p /etc/todo
+	[ -e /etc/todo/config ] || \
+		sed "s/^\(export[ \t]*TODO_DIR=\).*/\1~\/.todo/" todo.cfg > /etc/todo/config
 #
 # Testing
 #