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
```

Start at the `ionic start` command and inspect how the existing Ionic 4 beta starters and their options are selected. Compare the requested flags and prompts with related issues 3490 and 4236. Done means one starter can expose configurable options such as navigation style, testing, theme, lazy loading, and CSS.

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