Add a way for offline installation

[tychenjiajun]

It seems that yarn install sass-embedded will download embedded-protocol and dart-sass-embedded from Github, which makes it impossible to succeed in a network that has no Github access?

[nex3]

We download these files on installation because we don't know until installation-time which specific operating system x architecture combination you'll need. I don't think it's feasible for us to bundle all of the embedded executables with the npm package, so how else could we make them available?

[tychenjiajun]

In node-sass, we have binary configuration parameters. Can we have a similar config in this project?

[nex3]

Specifying a proxy server isn't really "offline" so much as "online from a different place". I think #117 is what you want, which will make Dart Sass follow npm's conventions for locating proxy servers.