Feature suggestion: option to deploy to different endpoint

[zephyr-wong]

It will be useful to separate deployments to dev and prod server. It may be achieved by specifying the config file used, like skycli deploy --config skygear.dev.json (ref: https://webpack.js.org/api/cli/#usage-with-config-file). Or, skycli deploy --dev with a, say devApp, property in skygear.json.