[core] Add per-module interval configuration

Allow individual modules to set different update intervals by using an
overriding 'interval' parameter.

see #574

core/module.py

@@ -36,6 +36,7 @@ class Module(core.input.Object):
         for widget in self._widgets:
             widget.module(self)
         self._name = None
+        self.next_update = None
 
     def parameter(self, key, default=None):
         value = default

core/output.py

@@ -102,10 +102,15 @@ class i3(object):
         return widgets
 
     def update(self, affected_modules=None):
+        now = time.time()
         for module in self._modules:
             if affected_modules and not module.id() in affected_modules:
                 continue
+            if not affected_modules and module.next_update:
+                if now < module.next_update:
+                    continue
             module.update_wrapper()
+            module.next_update = now + float(module.parameter('interval', self._config.interval()))
             for widget in module.widgets():
                 self._status[widget] = widget.full_text()