Add 'contest' mode

[RagnarGrootKoerkamp]

Teams should be able to run this is contest mode:

• start a contest and write config to a file: #problems, duration of contest
• run code on samples, print output -> wrapper around test
• run code on all data, recording AC/WA and submission time
• print contest stats: solve times, penalty, problems solved
• optionally: end contest and print final stats.