Add tuple of cell types to nbformat API

[wstomv]

When using nbformat as a library to process Jupyter notebooks, it would be convenient to have a tuple (or list, or set) of allowed cell types. For instance,

CELL_TYPES = ('markdown', 'code', 'raw', )

It would also be useful to have some other similar constants on the API, such as

OPTIONAL_CELL_FIELDS = {
    'markdown': {"attachments"},
    'code': set(),
    'raw': {"attachments"}
}
OPTIONAL_OUTPUT_TYPES = {
    'execute_result': {'data', 'metadata' ,'execution_count'},
    'stream': {'name', 'text'},
    'display_data': {'data', 'metadata', },
    'error': {'ename', 'evalue', 'traceback'},
}