Generate .gitignore for new project.

While setting up my first helios app on Heroku, I found that the `helios server` command will create `.sass-cache` folder.

Thus I added a `.gitignore` template and also make sure that `helios new` won't generate `.gitignore` file while `--skip-git option` passed.

Thanks for your nice work. You always created something amazing. :+1: