Proposal: Adding pg_engine flag

[hzlmn]

Would be nice if tool provide ability to choose postgres engine (aiopg or asyncpg).
Possible command could looks like

create-aio-app project --pg_engine asyncpg

If you consider it as an option i could send pr.

[aio-libs-bot]

GitMate.io thinks possibly related issues are #24 (add wait script for redis), #1 (Add flake8 and travisci builds), #8 (Add test for the cookiecutter template), and #21 (Add Procfile for deploying to Heroku).

[Arfey]

I think it's good idea. I will wait for your pr 😀

[hzlmn]

There is current limitation in cookiecutter to add conditional options like if we check use_postgres only then we should choose postgres_engine option. So maybe it will be easier to combine this too options into one.