Added external system commands support for events

Print start, print end, print cancel, and Z  height change can now
trigger external system commands.  These are set in config.yaml and take
_FILE_, _PERCENT_, _LINES_ and _ZHEIGHT_  tokens to be passed to
external commands.

system_commands:
  cancelled: echo cancelled _FILE_ at _PROGRESS_
percent done.
  print_done: echo done with _FILE_
  print_started: echo
starting _FILE_
  z_change: echo _LINE_ _PROGRESS_ _ZHEIGHT_

octoprint/server.py

@@ -375,7 +375,15 @@ def getSettings():
 		},
 		"system": {
 			"actions": s.get(["system", "actions"])
+		},
+        "system_commands": {
+			"print_done": s.get(["system_commands", "print_done"]),
+			"cancelled": s.get(["system_commands", "cancelled"]),
+			"print_started": s.get(["system_commands", "print_started"]),
+			"z_change": s.get(["system_commands", "z_change"])
+			
 		}
+
 	})
 
 @app.route(BASEURL + "settings", methods=["POST"])
@@ -417,6 +425,12 @@ def setSettings():
 		if "system" in data.keys():
 			if "actions" in data["system"].keys(): s.set(["system", "actions"], data["system"]["actions"])
 
+		if "system_commands" in data.keys():
+            if "z_change" in data["system_commands"].keys(): s.set(["system_commands", "z_change"], data["system_commands"]["z_change"])
+            if "print_started" in data["system_commands"].keys(): s.set(["system_commands", "print_started"], data["system_commands"]["print_started"])
+            if "cancelled" in data["system_commands"].keys(): s.set(["system_commands", "cancelled"], data["system_commands"]["cancelled"])
+            if "print_done" in data["system_commands"].keys(): s.set(["system_commands", "print_done"], data["system_commands"]["print_done"])
+            
 		s.save()
 
 	return getSettings()

octoprint/settings.py

@@ -69,8 +69,14 @@ default_settings = {
 	"controls": [],
 	"system": {
 		"actions": []
-	}
-}
+	},
+    "system_commands": {
+        "z_change":None,
+        "print_started":None,
+        "cancelled":None,
+        "print_done":None
+    }
+} 
 
 valid_boolean_trues = ["true", "yes", "y", "1"]