[core/output] Add widget drawing

Add basic drawing of widgets. Each module instance returns a list of
widgets using the widgets() method which is then forwarded to the draw()
method of the configured output.

see #23

bumblebee/output.py

@@ -18,4 +18,15 @@ class I3BarOutput(object):
         """Finish i3bar protocol"""
         sys.stdout.write("]\n")
 
+    def draw(self, widgets):
+        """Draw a number of widgets"""
+        if not isinstance(widgets, list):
+            widgets = [widgets]
+        result = []
+        for widget in widgets:
+            result.append({
+                u"full_text": widget.text()
+            })
+        sys.stdout.write(json.dumps(result))
+
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