Apply config file keys to sub commands

[kumar303]

This is a stepping stone toward implementing all of #176

After #728, let's allow a config file to define both global options and sub-command options like this:

module.exports = {
  sourceDir: './extension',
  sign: {
    apiKey: process.env.MY_API_KEY,
    apiSecret: process.env.MY_API_SECRET,
  },
  run: {
    firefoxBinary: 'nightly',
  },
};

Any sub-command option should be named after the command (like sign, run, lint, etc).