ons and arguments.
    cmd is asciidoc command or asciidoc.py path.
    opts and args conform to values returned by getopt.getopt().
    Raises SystemExit if an error occurs.

    Doctests:

    1. Check execution:

       >>> infile = io.StringIO('Hello *{author}*')
       >>> outfile = io.StringIO()
       >>> opts = []
       >>> opts.append(('--backend','html4'))
       >>> opts.append(('--no-header-footer',None))
       >>> opts.append(('--attribute','author=Joe Bloggs'))
       >>> opts.append(('--out-file',outfile))
       >>> execute(__file__, opts, [infile])
       >>> print(outfile.getvalue())
       <p>Hello <strong>Joe Bloggs</strong></p>

       >>>

    r