Use configured publicPath in watch

[dangreaves]

This PR updates the getPublicPath utility to return the configured publicPath during watch mode as well as the currently configured build mode.

My assumption on why getPublicPath exists is that when using the dev server via poi develop, assets are usually served from / instead of the configured asset path. For this reason, I have kept the develop command excluded from the list.

For anyone wishing to override the publicPath across all commands, they can use the chainWebpack hook to force update the configuration.

chainWebpack(config, context) {
  config.output.publicPath("/assets/");
}

This PR should somewhat resolve #460.