feat(cli): various cli improvements

[franklevasseur]

This PR includes:

1. the download command is now a subcommand of lang: ./nlu lang download --dim 100 --lang fr
2. all default config values were moved at a single location
3. every cli parameter can be replaced by an env variable instead. Environment variables are generated from yargs schema builder
4. json config file now has a schema that is generated from yargs schema builder
5. subcommands init (./nlu init && ./nlu lang init) generate empty json configuration files with a schema in app data

Here's the priority order in case of conflict:

cli arg > config file > env variable