Add UE4CLI_QUIET env var for silencing stderr logging

Author: adamrehn

ue4cli/Utility.py

@@ -20,7 +20,8 @@ def printStderr(*args, **kwargs):
 		"""
 		Prints to stderr instead of stdout
 		"""
-		print(*args, file=sys.stderr, **kwargs)
+		if os.environ.get('UE4CLI_QUIET', '0') != '1':
+			print(*args, file=sys.stderr, **kwargs)
 
 	@staticmethod
 	def readFile(filename):