Add support to enable asyncio REPL in flask shell command

[yuxiaoy1]

As flask and sqlalchemy have async support now, add support to enable asyncio REPL in flask shell command would be more convienient to test async code.
Like python3 -m asyncio enables asyncio REPL, something like flask shell --async would make sense.
What do you think?

[yuxiaoy1]

@davidism Hello David, how about the suggestion?