Add player stat tracking

[suve]

It would be cool to have the game track some player stats and then display them on the victory screen.

Some stats that could be collected:

• Play time
• Number of deaths
• Hits taken / total damage taken
• Shots fired / hits made / accuracy
• Enemies defeated / total damage made
• Enemies killed by contact

[suve]

Added in bef002e2648bbf66e9444cbc51775d8a277d7085.