Made verbose message of _addto() consistent with other task messages.

In verbose mode, commands first print the updated task, then summarize the change in the following line.
The add/addm/addto commands deviated from this, using "TODO: '<task>' added on line N."
This change splits this into two separate lines, obsoletes the implementation jargon of "line", and makes it easier to visually parse the message.
This is also a preparation for a possible future use of a _list()-like function, so that the updated task is processed and highlighted like the normal task output.

todo.sh

@@ -484,7 +484,8 @@ _addto() {
         TASKNUM=$(sed -n '$ =' "$file")
         BASE=$(basename "$file")
         PREFIX=$(echo ${BASE%%.[^.]*} | tr [a-z] [A-Z])
-        echo "${PREFIX}: '$input' added on line $TASKNUM."
+        echo "$TASKNUM: $input"
+        echo "${PREFIX}: $TASKNUM added."
     }
 }