Overview: HTML engine

[eliot-akira]

There's a new HTML engine, based on unified and hast. It has a similar architecture like PostCSS, a pipeline of transformations with each feature as a plugin. It's much easier to customize the details and extend the syntax.

• Replace Prettier's formatter based on Angular HTML parser #99
• Save pre-parsed templates to improve performance #100
• Structural editor: Switch between code and tree view for editing visually #107