feat: add options.dest

Add a dest option to generate the constant file to a specific path or to change the default file name

This options is available in the grunt plugin. I plan to migrate from grunt to gulp and this option will be nice :)
Our json config files are suffixed by the target environment, in the same folder. At the moment, a file placed in /config/config_dev.json can not be generated to /src/app/config.js.