Provide a way to skip formatting on the whole file

[fabioz]

I'd like to know if it would be possible to provide a marker in the document to skip formatting on the whole file.

isort has a feature where putting a comment with isort:skip_file to the file makes the formatting not be applied, ideally I'd like to have something as autopep8:skip_file too (although any other marker would do too).

[hhatto]

I think that there is no need therefore can be replaced by --exclude option.