color: set initial default value for 'colormode' to None

This should not introduce any behavior changes when using the color extension.
In practive, the colormode will be setup at early at run time to the proper
value (from config and environment).

We do this change as this gets us closer of a state were we can have all the
mechanisms associated to color in core with the feature disabled by default.

hgext/color.py

@@ -297,7 +297,7 b' def _modesetup(ui, coloropt):'
     return None
 
 class colorui(uimod.ui):
-    _colormode = 'ansi'
+    _colormode = None
     def write(self, *args, **opts):
         if self._colormode is None:
             return super(colorui, self).write(*args, **opts)