Add support for custom judging functions.

[xtevenx]

Some problems require a judging function made specifically for that problem. For example: "find a prime factor of a number". In the example, any of the factors of the number will work so we cannot do a simple string comparison like the current judges.

A possible implementation is to add another parameter in the problem specifications file which contains the Python code for the judge function and load it with exec when we are judging. To signify that there is a non-standard judge function, the judge parameter in the problems specifications file can be set to something like custom.