Countdown persistence and modifiability

[gaboss44]

• Exposed Countdown secondsLeft in Countdown#getSecondsLeft and Countdown#setSecondsLeft.
• Added countdown int getter in RMine, save methods and usage of them on unload.
• Added countdown config key (reset.time.countdown).
• Mine countdown persists after reload and/or server restart.
• Added setcountdown and resetcoutdown commands.
• Added language keys for such commands.

[neziw]

seems fine for me, however it should be tested before merging I think, @joserodpt

[joserodpt]

nice!

[joserodpt]

thank you so much @gaboss44 for the PR. feel free to do more in the future :)