Ensure that markup is a tuple.

`self.markup` is a list when using the `-m|--markup` cli option, but testing the
extension with `endswith` works only with tuples.

pelican/generators.py

@@ -96,7 +96,7 @@ class Generator(object):
             extensions are allowed)
         """
         if extensions is None:
-            extensions = self.markup
+            extensions = tuple(self.markup)
         basename = os.path.basename(path)
         if extensions is False or basename.endswith(extensions):
             return True