[core] Allow modules to specify default update interval

Expanding on the implementation in d582016, add a decorator
`core.module.every()` that allows a module to specify how often to
update the module's state.

This can still be overridden using the CLI parameter `interval`.

modules/core/kernel.py

@@ -8,6 +8,7 @@ import core.module
 import core.widget
 
 class Module(core.module.Module):
+    @core.module.every(minutes=60)
     def __init__(self, config=None):
         super().__init__(config, core.widget.Widget(self.full_text))