[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/github.py

@@ -25,6 +25,7 @@ class Module(bumblebee.engine.Module):
                                      bumblebee.output.Widget(full_text=self.github)
                                     )
         self._count = 0
+        self.interval_factor(60)
         self.interval(5)
         self._requests = requests.Session()
         self._requests.headers.update({"Authorization":"token {}".format(self.parameter("token", ""))})