Add keywords and npm_install prompts, two helpers

Hello,

I have added:
* two built-in prompts: `keywords` and `npm_install`
* `prompt.sanitizeYesNo` to convert `yes/no` answers into boolean values
* `init.runNpmInstall` to run `npm install` command
* basic support for `github_user` key in `defaults.json`
* support for project’s name as default repository name