Ask for password only once it is needed

[tsloughter]

Currently if there is an issue with the package being published you might not realize it, or be told, until after it asks for you password, which is a bit annoying. mix doesn't ask for password until it is needed.

[starbelly]

Agreed.

[starbelly]

I believe the best way to tackle this, and perhaps why it was not tackled before it was need a simple process that creates an ets table such that we do not need to muck around with passing a config map everywhere. That seems like the neatest approach to me, and is what mix hex does (via Agent). Unless, there are objections, I'll look at turning rebar3_hex_config into a process.

For good measure we should start slinging around the decrypted api key in a fun such that it doesn't end up leaked on the screen.