Implement #34 (option to render templates with surrounding HTML comments indicating Twig's file path)

[MichaelAllenWarner]

• Implements #34
• Adds a type-test for the new feature
• Updates the README accordingly

[MichaelAllenWarner]

Closed temporarily; implemented incorrectly.

[MichaelAllenWarner]

Closed temporarily; implemented incorrectly.

Never mind — the problem I encountered appears to be elsewhere. Re-opening.

[MichaelAllenWarner]

This is better implemented as a separate Webpack loader that runs before Twing-loader.