Add simple sprinting mod

Based on [sprint] by GunshipPenguin

mods/PLAYER/mcl_sprint/README.md

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+Sprint Mod for MineClone 2
+Forked from [sprint] by GunshipPenguin  
+
+Allows the player to sprint by either double tapping w or pressing e. 
+By default, sprinting will make the player travel 80% faster and 
+allow him/her to jump 10% higher. 
+
+Licence: CC0 (see COPYING file)
+
+---
+
+This mod can be configured by changing the variables declared in 
+the start of init.lua. The following is a brief explanation of each 
+one.
+
+SPRINT_METHOD (default 1)
+
+What a player has to do to start sprinting. 0 = double tap w, 1 = press e.
+Note that if you have the fast privlige, and have the fast 
+speed turned on, you will run very, very fast. You can toggle this 
+by pressing j.
+ 
+SPRINT_SPEED (default 1.5)
+ 
+How fast the player will move when sprinting as opposed to normal 
+movement speed. 1.0 represents normal speed so 1.5 would mean that a 
+sprinting player would travel 50% faster than a walking player and 
+2.4 would mean that a sprinting player would travel 140% faster than 
+a walking player.
+
+SPRINT_JUMP (default 1.1)
+
+How high the player will jump when sprinting as opposed to normal 
+jump height. Same as SPRINT_SPEED, just controls jump height while 
+sprinting rather than speed.
+
+SPRINT_STAMINA (default 20)
+
+How long the player can sprint for in seconds. Each player has a 
+stamina variable assigned to them, it is initially set to 
+SPRINT_STAMINA and can go no higher. When the player is sprinting, 
+this variable ticks down once each second, and when it reaches 0, 
+the player stops sprinting. It ticks back up when the player isn't 
+sprinting and stops at SPRINT_STAMINA. Set this to a huge value if 
+you want unlimited sprinting.
+
+SPRINT_TIMEOUT (default 0.5)
+
+Only used if SPRINT_METHOD = 0.
+How much time the player has after releasing w, to press w again and 
+start sprinting. Setting this too high will result in unwanted 
+sprinting and setting it too low will result in it being 
+difficult/impossible to sprint.