Keep track of PvP scores somewhere

[allejo]

When _ltsResetScoreOnElimination is true, scores are reset every round but due to that, you lose your overall score and also PvP stats too.

Requirements

• Introduce new /playerstats command to display your current PvP scores against other players (even eliminated players)
  • Additionally, allow /playerstats <id or callsign> to see the stats of another player
• Update ending announcement to have aggregate K/D score from all rounds