color: fallback and test label as an effect

With this changeset, we can now send a color name directly to the label
function, e.g.

$ hg log -r . -T "{label('red', node|short)}\n"

hgext/color.py

@@ -382,6 +382,8 class colorui(uimod.ui):
             s = _styles.get(l, '')
             if s:
                 effects.append(s)
+            elif valideffect(l):
+                effects.append(l)
         effects = ' '.join(effects)
         if effects:
             return '\n'.join([render_effects(s, effects)