Option to output a json file per locale

Having all translations in a single file makes it larger to download. A file per locale would allow just the relevant translations to be used

`vue-webpack-gettext` implements this as a `--multiple` option: https://github.com/kennyki/vue-webpack-gettext/blob/437f570e1a1eff59e299e9fbcda644e138a31ee5/compile.js#L48