[themes] Make individual items theme-able (a bit)

Individual items in the bar can now be configured with a prefix and a
suffix. It works like this:

* If there is a specific module configuration in the theme
  configuration, use that (i.e. { "<modulename>": { "prefix: " a " } })
* Otherwise, if there is a configuration in the "default" section of the
  theme, use that
* Otherwise, if the module object itself has a method called like the
  required attribute (prefix, suffix), use that
* Otherwise, leave prefix/suffix empty ("")

bumblebee/module.py

@@ -6,4 +6,10 @@ class Module(object):
     def theme(self):
         return self._theme
 
+    def data(self):
+        pass
+
+    def state(self):
+        return "default"
+
 # vim: tabstop=8 expandtab shiftwidth=4 softtabstop=4