[modules] Initial module loading framework

Add a very simplistic framework for loading modules that query system
data. The user can provide a list of modules via an argument switch, and
the name of the module is used to look up a Python module that has to
have a class called "Module".

The outer framework (the outputs, in particular) then uses various query
methods of that class to construct a meaningful output.

bumblebee/outputs/i3.py

@@ -1,13 +1,22 @@
 import json
 
 class i3bar:
-    def preamble(self):
+    def __init__(self):
+        self._data = []
+    def start(self):
         return json.dumps({ "version": 1 }) + "["
 
-    def data(self, data):
-        return json.dumps(data) + ","
+    def add(self, obj):
+        self._data.append({
+            "full_text": obj.data()
+        })
 
-    def finalize(self):
+    def get(self):
+        data = json.dumps(self._data)
+        self._data = []
+        return data + ","
+
+    def stop(self):
         return "]"
 
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