run: prompt user to save api keys

[matildepark]

Fixes #61.

Rewrites some config resolution code to be a bit nicer to look at; adds some utilities to check for and merge two potential configs (the ones we pipe in as a flag, and the .nolitarc), and prompts the user to save to .nolitarc if they're entering HDR or agent API keys.

It's a little annoying because we take in so many possible options:

• Priority one is whatever we specify as a flag. It's like the equivalent of CSS inline styling.
• Priority two is whatever we specify as a config.json, via a flag. It's higher priority than background vars.
• Priority three is our .nolitarc settings.
• Priority four is process.env variables.

All these need to be merged into one big config as we go, mutating it as we ask the user questions.

In the process I also fixed a bug prohibiting piping the output -- we just need to console.log it when we're done.

[matildepark]

Tested a few cases on Mac and Windows now. Needed to nestle the request to save the key into the promise, but only on Mac? Well, okay. We pass on requesting to save if there's nothing entered. Removed the call to dashboard, which will always error out if anyone tries to sign up.

• Tried without any nolitarc. Fine.
• Tried partial nolitarc. Fine.
• Tried injecting a process.env var and then also using config.json. Fine.

I have reasonable belief that the priority list above is being respected in mixed configuration edge cases, so will place into dev.