HTML partial support

[k0ff33]

As mentioned in #889, provides a basic ability to separate big and ugly html files into smaller partials, without adding any complicated template engine.

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[k0ff33]

I signed it!

[googlebot]

CLAs look good, thanks!

[JimmyMultani]

Any news on this?

[Victory]

I pulled these changes into my project and it worked a charm.

+1 to add this to main branch.

[sbarfurth]

I would like to suggest that instead of constraining partials into a folder, they could instead be prefixed with an underscore. This would leave more room with folder structure and seems compliant with what is usually done with Sass.