[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`.

core/module.py

@@ -12,6 +12,15 @@ except Exception as e:
 
 log = logging.getLogger(__name__)
 
+def every(minutes=0, seconds=0):
+    def decorator_init(init):
+        def call_init(obj, *args, **kwargs):
+            init(obj, *args, **kwargs)
+            if obj.parameter('interval') is None:
+                obj.set('interval', minutes*60 + seconds)
+        return call_init
+    return decorator_init
+
 def load(module_name, config=None):
     error = None
     for namespace in [ 'core', 'contrib' ]:

doc/HOWTO_MODULE.md

@@ -35,3 +35,11 @@ class Module(core.module.Module):
 
 # vim: tabstop=8 expandtab shiftwidth=4 softtabstop=4
 ```
+
+## `update` vs. `full_text`
+TODO
+
+## TODOs
+- default update interval
+- scrolling
+- 

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))