add sort_when_creating_position option

[alberto-bottarini]

Currently, there's no direct way to automatically move a newly created record to the top position within the sorting system.
I've added sort_when_creating_position property to the sortable model (and in the config), allowing the newly created record to be moved to the top position.

[freekmurze]

Since not too many people will probably need this, I'm going to pass on it in order to keep the package simple.

Thanks for your work on this!