Add `config` option

[luisvt]

As a developer I would like to have the ability to generate a file generator.config.yaml for projects that has been started previously without this generator.

So that I would like to run:

ng2gen config

and then the program should ask me some questions like:

• would you like to use css preprocesor Sass, Less, Stylus, Css or None:
• would you like to use toolkit ng_bootstrap, n2s_strap, ng-prime, ng-material, ...
• ...

then generator should save those options into generator.config.yaml. Finally whenever I run other commands, they should pick those options.

[lejard-h]

humm weird, I already did that but it's gone, don't know why :D

[luisvt]

If you did, most likely I removed by mistake.

[luisvt]

So if you can, please add the changes.