define 'iterable' = None in tg_tqdm()

[metya]

Define parameter 'iterable' is None by default in tg_tqdm().

It is makes possible operate with tg_tqdm like as tqdm original with 'with' statement.
For example:

with tg_tqdm(token=example_bot_token, chat_id=example_chat_id, total=100) as t:
    while some_statement is True:
        do_someting():
        t.update()

It is really can be useful with some unknown action that not iterable itself of don't have length of itself.

Otherwise, it was necessary to do it like:

with tg_tqdm(iterable = range(1), token=example_bot_token, chat_id=example_chat_id, total=100) as t:
    while some_statement is True:
        do_someting():
        t.update()