Allow explicit disabling of extensions

If the first character of an extension path is '!', the extension
is silently skipped.

mercurial/extensions.py

@@ -70,6 +70,8 b' def loadall(ui):'
     result = ui.configitems("extensions")
     for i, (name, path) in enumerate(result):
         if path:
+            if path[0] == '!':
+                continue
             path = os.path.expanduser(path)
         try:
             load(ui, name, path)