Implements #27 among other things

It was found that the Spigot API's methods for cancelling player collisions won't work without changing the scoreboard. Because of that, the normal option has been disabled. The invisibility option has also been removed, as that's a bad idea if players can still push each-other.
The toggle player option which is implemented in this commit does disable player collision, so that's the only working way right now.

A potential ConcurrentModificationException has been fixed.
The parkourCheckpoint command has been removed, as the functionality is now available through the API.

src/main/resources/plugin.yml

@@ -150,13 +150,6 @@ commands:
     permission: minigames.remove.parkour
     usage: /<command> <arena>
     description: Used to remove an existing parkour arena
-  parkourCheckpoint:
-    aliases:
-      - pcheckpoint
-      - pcheck
-    permission: minigames.join.parkour
-    usage: /<command>
-    description: Used to teleport to the previous checkpoint while in a parkour arena
 permissions:
   minigames.*:
     description: Gives all permissions