(Feature Request) - Accessibility Testing support

### Bug Report or Feature Request (mark with an `x`)
```
- [x ] feature request
```

### Desired functionality.
**Accessibility testing**

Is it possible to include tools like [axe](https://github.com/dequelabs/axe-cli) or [pa11y](https://github.com/pa11y/pa11y)
for accessibility testing into the CLI?

Start by reviewing the Angular CLI's existing testing and integration entry points, then compare the proposed axe and pa11y tools. Define which accessibility checks the CLI should support, how users would invoke them, and what tests or documented results would establish that the feature is complete.

Written by the indexing model from the issue text.