[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/themes/default.json

@@ -0,0 +1,11 @@
+{
+	"defaults": {
+		"prefix": " ",
+		"suffix" : " "
+	},
+	"battery": {
+		"prefix": "bat "
+	},
+	"time": {
+	}
+}

bumblebee/themes/simple.json

@@ -1,3 +0,0 @@
-{
-
-}