#71 Warn when output or diff is truncated

[Bruce Merry]

If the mark files are too large they get truncated. Even if expected output is within the size limit, user output and the diff can be much larger. This can lead judges to think that test cases have been omitted (format error) when in fact they've just been truncated. Mark files that have been truncated should have some sort of warning on them, either as extra text ("[file truncated at xxx bytes]") or as a separate signal in the GUI.

The default of 64KB can probably also be doubled.