Add achievements

[ChrisNeedham24]

Cool idea for special things that players can achieve. Should be achievements of varying levels of difficulty. For example, Win a game is an easy one, but you could also have Win an elimination game with 10+ players.

[ChrisNeedham24]

Will want to store these in statistics.json.

[ChrisNeedham24]

I'm going to stick with achievements that can all be calculated at the same time after a turn has ended, so the below ideas won't be able to be done:

• Work on each project at least once in one game (can be multi-settlement)
• Move a unit over 50 quads in a deployer unit
• Kill a heathen during nighttime
• Attack an enemy unit and die yourself
• Take an enemy settlement but unit dies
• Take a settlement as the concentrated

Maybe these can be introduced at a later date.

[ChrisNeedham24]

~We also can't do the achievement 'Free For All' (Win a game with 14 players" because of how the end_turn() function works. This is because we never know when a player has actually achieved a victory. Separate handling could be added in save_stats_achievements() for this, but that's not something I want to do at this time.~

~The same applies for 'Speed Run' (Win a 2 player game in 25 turns or less).~

Realised that that would also mean none of the faction-specific victory achievements would work, so implementing now.