Rename default inbox from todo.md to TODO.md

Because it is more common:
- https://github.com/todomd/todo.md
- https://github.com/todo-md/todo-md

mdt

@@ -233,7 +233,7 @@ die() {
 # If inbox isn't specified, use default. 
 # If it doesn't exist, ask to create.
 set_inbox() {
-  _default_inbox="${dir}/todo.md"
+  _default_inbox="${dir}/TODO.md"
   inbox="${inbox:-$_default_inbox}"
 
   if [ ! -f "${inbox}" ]; then