While I dont know of a software that does it automatically. You can use "Nutshell Mail" to keep a track of users you who follow and unfollow you everyday. You get this in a nice email summary. You can send this message to the ones who unfollowed you manually.
So this does half the job, i.e. of tracking users who are unfollowing you but doesnt delivery automated DM's.
On another note, once a user has unfollowed you, I dont see much value in sending them a DM to request them to come back, most wont.