Added a new variable $TODO_FULL_SH

This gives access to the full path of the script. It may prove useful
for addon developers to rerun todo.sh from within their addons, in order
to populate variables for context, projects etc. As we have the config
and actions directory available via variables, this is a missing piece.

todo.sh

@@ -17,9 +17,10 @@ EndVersion
     exit 1
 }
 
-# Set script name early.
+# Set script name and full path early.
 TODO_SH=$(basename "$0")
-export TODO_SH
+TODO_FULL_SH="$0"
+export TODO_SH TODO_FULL_SH
 
 oneline_usage="$TODO_SH [-fhpantvV] [-d todo_config] action [task_number] [task_description]"