[core/theme] Add support for foreground and background colors

Themes can now define "fg" and "bg" attributes that are used for
foreground (text) color and background color.

see #23

bumblebee/output.py

@@ -53,7 +53,9 @@ class I3BarOutput(object):
         if suffix:
             full_text = u"{}{}".format(full_text, suffix)
         self._widgets.append({
-            u"full_text": u"{}".format(full_text)
+            u"full_text": u"{}".format(full_text),
+            "color": self._theme.fg(widget),
+            "background": self._theme.bg(widget),
         })
 
     def begin(self):

bumblebee/theme.py

@@ -30,6 +30,14 @@ class Theme(object):
         """Return the theme suffix for a widget's full text"""
         return self._get(widget, "suffix", None)
 
+    def fg(self, widget):
+        """Return the foreground color for this widget"""
+        return self._get(widget, "fg", None)
+
+    def bg(self, widget):
+        """Return the background color for this widget"""
+        return self._get(widget, "bg", None)
+
     def loads(self, data):
         """Initialize the theme from a JSON string"""
         theme = json.loads(data)