Run markdownlint on matched_filenames.

def _run_markdownlint(matched_filenames, show_lint_files):
    """Run markdownlint on matched_filenames."""
    from prospector.message import Message, Location

    for filename in matched_filenames:
        _debug_linter_status("mdl", filename, show_lint_files)

    try:
        proc = subprocess.Popen(["mdl"] + matched_filenames,
                                stdout=subprocess.PIPE,
                                stderr=subprocess.PIPE)
        lines = proc.communicate()[0].decode().splitlines()
    except OSError as error:
        if error.errno == errno.ENOENT:
            return []

    lines = [
        re.match(r"([\w\-.\/\\ ]+)\:([0-9]+)\: (\w+) (.+)", l).groups(1)
        for l in lines
    ]
    return_dict = dict()
    for filename, lineno, code, msg in lines:
        key = _Key(filename, int(lineno), code)
        loc = Location(filename, None, None, int(lineno), 0)
        return_dict[key] = Message("markdownlint", code, loc, msg)

    return return_dict