Pass run_once to ConfigLoader, make it respect run_once

It only respects run_once by using DummyWatcher instead of a watcher doing 
something potentially useful

Ref #711

powerline/__init__.py

@@ -121,7 +121,7 @@ class Powerline(object):
 			'load_theme': True,
 		}
 		self.shutdown_event = shutdown_event or Event()
-		self.config_loader = config_loader or ConfigLoader(shutdown_event=self.shutdown_event)
+		self.config_loader = config_loader or ConfigLoader(shutdown_event=self.shutdown_event, run_once=run_once)
 		self.run_loader_update = False
 
 		self.renderer_options = {}

powerline/lib/config.py

@@ -20,11 +20,22 @@ def load_json_config(config_file_path, load=json.load, open_file=open_file):
 		return load(config_file_fp)
 
 
+class DummyWatcher(object):
+	def __call__(self, *args, **kwargs):
+		return False
+
+	def watch(self, *args, **kwargs):
+		pass
+
+
 class ConfigLoader(MultiRunnedThread):
-	def __init__(self, shutdown_event=None, watcher=None, load=load_json_config):
+	def __init__(self, shutdown_event=None, watcher=None, load=load_json_config, run_once=False):
 		super(ConfigLoader, self).__init__()
 		self.shutdown_event = shutdown_event or Event()
-		self.watcher = watcher or create_file_watcher()
+		if run_once:
+			self.watcher = DummyWatcher()
+		else:
+			self.watcher = watcher or create_file_watcher()
 		self._load = load
 
 		self.pl = None