Change the way we're dealing with "backup" file

Currently, when running mat2 on myfile.jpg, two files are outputted:

• myfile.jpg: the original file
• myfile.cleaned.jpg: the cleaned file

I think that it would make more sense to have this instead:

• myfile.jpg: the cleaned file
• myfile.jpg.bak: the original file

The main drawback is that if the cleaning process fails, the user will be left with a myfile.jpg.bak and might wonder where their file is.

The reason why I'm suggesting this change is that some users have been confused by the current scheme, and I think that the new one makes more sense.

Any opinions?