Extracts value for the specified metadata key from the given extension set.

    Keyword arguments:
    source -- string containing MultiMarkdown text
    ext -- extension bitfield for processing text
    key -- key to extract

def value(source, key, ext=COMPLETE):
    """Extracts value for the specified metadata key from the given extension set.

    Keyword arguments:
    source -- string containing MultiMarkdown text
    ext -- extension bitfield for processing text
    key -- key to extract
    """
    _MMD_LIB.extract_metadata_value.restype = ctypes.c_char_p
    _MMD_LIB.extract_metadata_value.argtypes = [ctypes.c_char_p, ctypes.c_ulong, ctypes.c_char_p]

    src = source.encode('utf-8')
    dkey = key.encode('utf-8')

    value = _MMD_LIB.extract_metadata_value(src, ext, dkey)
    return value.decode('utf-8') if value else ''