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`.