[core] Switch update interval parameter to seconds

As correctly pointed out by @rrhuffy, restricting update intervals to
minutes is pretty arbitrary. Therefore, change logic to specify updates
intervals in *seconds*.

Also, to maintain backwards compatibility for users that already have
custom intervals in their config, allow a module to specify their own
"factor" for intervals. So a module that expects the interval to be in
minutes can set the factor to 60.

bumblebee/modules/apt.py

@@ -44,6 +44,7 @@ class Module(bumblebee.engine.Module):
     def __init__(self, engine, config):
         widget = bumblebee.output.Widget(full_text=self.updates)
         super(Module, self).__init__(engine, config, widget)
+        self.interval_factor(60)
         self.interval(30)
 
     def updates(self, widget):