Add support for using cchardet to detect files' encoding
    
    cchardet is a much faster version of the chardet library that can
    be used to automatically detect the encoding of a file.
    
    Since this library is only available on python3, we're making it
    an optional dependency for now.
    
    Fixes https://pagure.io/pagure/issue/4977
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>