Refactor to move config to Shout home by default

This PR moves the logic for finding the config file into `helper.js`, causes the config to be looked for in the Shout home by default, and changes `shout config` to edit the file in the Shout home by default. `$SHOUT_CONFIG` can still override this.

This makes it easier to get going after an `npm install` (or similar installation process) without having to set environment variables or edit `config.js` in the source directory, as the config will default to `~/.shout/config.js`.