Graceful stop

[Dashboy1998]

Context

• Add in graceful stop

Choices

• Using rcon was mentioned by @thijsvanloef in Stopping does not appear to be graceful #62
• In the event rcon is not used then kill is used instead

Test instructions

1. Run with RCON_ENABLED=true
2. Join the game and move to a different location in game
3. Stop the container with ctrl + c
4. Restart container and join game again to verify it saved where you ended
5. Repeat 1 - 3 with RCON_ENABLED=false NOTE: this does not save without rcon

Checklist before requesting a review

• I have performed a self-review of my code
• I've added documentation about this change to the README.
• I've not introduced breaking changes.

[thijsvanloef]

Thank you for the PR! I will check it once I get home from work :D

[thijsvanloef]

Amazing work, made a small tweak, ready to merge :)