Sanitizer: Initial implementation

The AMP Toolbox should include a sanitizer that is a pure PHP variant of the collection of sanitizers currently found in the AMP for WordPress plugin.

The general idea is that the Sanitizer should take regular HTML5 markup and make smart transformations to turn it into AMPHTML markup.

- [ ] Build sanitizer pipeline, similar to optimizer pipeline (possible code reuse)
- [ ] Port over generic sanitizers from https://github.com/ampproject/amp-wp
- [ ] Build CLI command to sanitize a URL or a piece of markup
- [ ] Optional: Build PSR-15 middleware to turn an HTML5 response into an AMPHTML response.

Start by reviewing the optimizer pipeline for possible reuse and comparing the generic sanitizers in the linked AMP for WordPress project. Define the scope across the sanitizer pipeline, URL or markup CLI command, and optional PSR-15 middleware. Done means the selected checklist items are implemented for transforming HTML5 markup into AMPHTML.

Written by the indexing model from the issue text.