Feat/create gulpset skeleton

[huyld]

In this PR

• Change the package name from gulpset-skeleton to create-gulpset-skeleton
• Create bootstrap script. It will
  • validate project name to follow npm naming rules and avoid conflict with existing packages.
  • copy skeleton files and directory from the package create-gulpset-skeleton into new project.
  • run yarn install to install all the dependencies listed within package.json.
• Update README.md

To-dos

I see some rooms for improvement:

• Validate project name to follow npm naming rules and avoid conflict with existing packages.
• Colorize console message. Currently log and error message are the same color.
• Isolate init script init.js. Currently it's in gulpset/. I think it should not be part of the skeleton.
• After copy skeleton files and dirs, init script should run yarn install to install all the dependencies listed within package.json.
• Generating package.json file for new project with devDependencies copied from gulpset-skeleton package.json. We cannot reuse the our package.json because of specific configuration such as name, author, license, etc.
• Update current README.md file of fourdigit/gulpset.