Add %T temp output filename format specifier

[zonaj]

With the %T format specifier, one is able to supply an output file for an action's result.
Example usage:

feh --action ';[flatten]convert %F -layers flatten %T' myimage.gif

This will provide an action that flattens myimage.gif (shows all its layers), writing out the result in a temporary file (the %T invents a unique temporary filename).

This patch is working out great for me, but I understand if it has issues and is not quite good enough to merge.