lesson-check: exclude aio.md, fix read_references

# Pattern to match reference links (to resolve internally-defined references).

P_INTERNAL_LINK_DEF = re.compile(r'^\[([^\]]+)\]:\s*(.+)')

# Pattern to match {% include ... %} statements

P_INTERNAL_INCLUDE_LINK = re.compile(r'^{% include ([^ ]*) %}$')

# What kinds of blockquotes are allowed?

KNOWN_BLOCKQUOTES = {

    'callout',

    {symbolic_name : URL}

    """

    if not ref_path:

        raise Warning("No filename has been provided.")

    result = {}

    urls_seen = set()

    if ref_path:

    with open(ref_path, 'r') as reader:

            for (num, line) in enumerate(reader):

                line_num = num + 1

        for (num, line) in enumerate(reader, 1):

            if P_INTERNAL_INCLUDE_LINK.search(line): continue

            m = P_INTERNAL_LINK_DEF.search(line)

                require(m,

                        '{0}:{1} not valid reference:\n{2}'.format(ref_path, line_num, line.rstrip()))

            message = '{}: {} not a valid reference: {}'

            require(m, message.format(ref_path, num, line.rstrip()))

            name = m.group(1)

            url = m.group(2)

                require(name,

                        'Empty reference at {0}:{1}'.format(ref_path, line_num))

                reporter.check(name not in result,

            message = 'Empty reference at {0}:{1}'

            require(name, message.format(ref_path, num))

            unique_name = name not in result

            unique_url = url not in urls_seen

            reporter.check(unique_name,

                           ref_path,

                               'Duplicate reference {0} at line {1}',

                               name, line_num)

                reporter.check(url not in urls_seen,

                           'Duplicate reference name {0} at line {1}',

                           name, num)

            reporter.check(unique_url,

                           ref_path,

                           'Duplicate definition of URL {0} at line {1}',

                               url, line_num)

                           url, num)

            result[name] = url

            urls_seen.add(url)

    return result

    (re.compile(r'index\.md'), CheckIndex),

    (re.compile(r'reference\.md'), CheckReference),

    (re.compile(r'_episodes/.*\.md'), CheckEpisode),

    (re.compile(r'aio\.md'), CheckNonJekyll),

    (re.compile(r'.*\.md'), CheckGeneric)

]