Implement aiohttp.web executable helper

[asvetlov]

python -m aiohttp.web HOST PORT pkg.module.main [ARGS]

See #690 for details

[asvetlov]

Questions:

1. How to support SSL context, do we need it at all?
2. How to skip HOST PORT parameters and use localhost 8080?
3. What pkg.module.main should be? I think it is either a regular function or await-able object (a coroutine or a Future) accepting ARGS. BTW I suggest pkg.module:main syntax to split import path and name inside imported module.
4. Do we want support path-to-file instead of import-path-to-module?
5. What's [ARGS] syntax? Is it just str params split by spaces or we want to use named parameters and apply type checking/conversions to ones?

[jashandeep-sohi]

1. Leave SSL out for now -- could always add a --ssl option later on that could create a SSLContext from a cert file.
2. argparse.ArgumentParser supports optional positional arguments. Might have to move pkg.module.main argument first though. ArgumentParser.parse_known_args might also be of use to avoid parsing [ARGS].
3. An async function that returns an web.Application instance.
4. ?
5. Use the leftover list of args returned by ArgumentParser.parse_known_args. The user can take care of the parsing in their main(args) implementation, if they need to.

[rutsky]

1. Perhaps do HOST PORT optional keyword arguments?

python -m aiohttp.web [--host HOST] [--port PORT] pkg.module.main [ARGS]

[asvetlov]

Looks good.
Any volunteer?
The PR should use already merged web.run_app().

[jashandeep-sohi]

I'll volunteer.

[asvetlov]

@jashandeep-sohi thank you too much. It will be significant addition, especially for newcomers.

I want to make aiohttp 0.21 release close to the end of January.
If you will not end the PR up to the date -- that's also fine, no rush. Looks like we have new release every month, 0.22 is expected in between of February last week end and March.

But, please, document the issue in PR.
You have much fluent English than me :)

[asvetlov]

Fixed by #740

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