Allow to pre-fetch NW.js distributions

[abvgdeyoj]

My proposal is:

1. Verb to download all the entries from nwjs-versions.json without doing anything else.
2. Introduce a directory to check for existing tar files before attemting to download

[Orochimarufan]

Sorry for the late response.
I'm not sure how downloading all versions would be useful to be honest, especially as more new versions are added. It might be useful to download specific versions without running anything however.
As for checking a directory, you can just extract the archive into the dist/ folder manually if you already have it. Downloads are currently never saved to disk as an archive.