Support for parsing HTML/CSS

[HadrienGardeur]

In its current state, the Go Toolkit is capable of parsing and extracting content from HTML resources using the Content Iterator API.

We'd like to go beyond that by working on:

• [ ] Support for generating Guided Navigation Documents from HTML
• [ ] Support for aria-details, aria-describedby and aria-labelledby

In addition to this extended work on HTML, we also want to introduce support for parsing CSS for both:

• [ ] External stylesheets
• [ ] Inline styles

A new command should allow users to extract Guided Navigation Documents as JSON, which will be the basis for future features of the toolkit (for example, automated generation of Media Overlays and/or audiobooks using TTS).