[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`.

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
+-