[themes] Add support for generic warning/critical colors

Font and background colors for warning and critical elements can now be
specified using fg-warning, fg-critical, bg-warning and bg-critical.

Also, optionally, the "urgent" flag will be set towards the i3bar, if
possible.

bumblebee/outputs/i3.py

@@ -20,6 +20,10 @@ class i3bar(bumblebee.output.Output):
             "color": theme.color(obj),
             "background": theme.background(obj),
         }
+
+        if theme.urgent(obj) and obj.critical():
+            data["urgent"] = True
+
         if theme.default_separators(obj) == False:
             data["separator"] = False
             data["separator_block_width"] = 0