[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/outputs/i3.py

@@ -9,8 +9,11 @@ class i3bar(bumblebee.output.Output):
         return json.dumps({ "version": 1 }) + "["
 
     def add(self, obj):
+        theme = obj.theme()
         self._data.append({
-            "full_text": obj.data()
+            "full_text": "%s%s%s" % (theme.prefix(obj), obj.data(), theme.suffix(obj)),
+            "separator": False,
+            "separator_block_width": 0,
         })
 
     def get(self):