Add shared storage for generators.

[jamen]

This is so sub-generators (or a collection of similarly-authored generators) can depend on shared data if needed... It relies on a module already in the dependency tree (dependency of a dependency) and is a small addition that enables a powerful subset style of generating, so I see no harm done.

Say for instance, I have foo, with foo:bar and foo:qux that both render different files that require name... If we run these at the same time:

$ yo foo:bar, foo:qux

You can prompt for the data one time (both generators foo:bar and foo:qux):

prompting: function() {
  if (!this.env.share.has('name')) {
    this.prompt({ ... }, function(res) {
      this.env.share.set('name', res.name);
    });
  }
}

Yet both generators can still retrieve the name later, with this.env.shared.get('name'), without having to reprompt the user.

The module I used, es6-map, was already in the dep tree, so I'm just reusing it here in a small implementation.

Closes yeoman/yo#416

[SBoudrias]

Hey, thanks for sending a PR. It is appreciated.

Unfortunately, this suggestion is not something I'm comfortable with going forward. Global values absent clearly defines structures always become a maintenance nightmare. This would impact users relying on this feature, but also our ability to push updates to our platform in the future.

[jamen]

Alright, no problem. It was worth a shot. :P