Configurable Starters

Many framework CLIs have a single starter with feature flags to enable/disable/switch certain features and preferences such as TypeScript, testing frameworks, theme, etc. We've already seen requests for this, such as https://github.com/ionic-team/ionic-cli/issues/3490, https://github.com/ionic-team/ionic-cli/issues/4236

The starters in place for Ionic 4 beta are the typical Ionic-style starters, but can we make one starter where the contents and options are configurable? Can `ionic start` prompt for all these options?

Examples:

```
ionic start --nav-style=tabs
ionic start --no-karma
ionic start --theme=dark
ionic start --no-lazy-loading
ionic start --css
```