Allow passing the tests' outs to checkers

[A-dead-pixel]

For some tasks a checker needs not only the input and the program's output, but also the output from out/. One such task is Przyciski from this OI. Extending the current format should be easy, but I suggest switching to sio2's checker format at least partially, that is passing in, userout and out filenames as args.

Additionally, imo toster should automatically handle test packages both with and without outs.

[MikolajKolek]

Hm, I'm not sure how I feel about passing in files, Toster will probably soon stop creating temporary output files for the tested program and switch entirely to doing all that in RAM as it's faster, this would require writing it back into a file, which would slow it back down. Adding some option to pass in the output and handling packages with and without outs does sound like a good idea though (even though I probably won't implement it in time to be useful for the current OI).

[A-dead-pixel]

I agree with the performance argument. I considered tmpfs, but it isn't a good enough solution, as it would require superuser privileges.