Embed external resources

[ArmanMazdaee]

What this PR does / why we need it:
Add support for embedding external resources in the rendered HTML so theme developers could place their styles and other external resources in external files

[stp-ip]

Overall it looks good. Should finish the full review tomorrow.

Some notes already that most likely should be morphed into issues, if you agree to handle longer term:

Make embedding default, but able to switch it off? That would allow someone to easily host it as a website and also make it easier for devs. By default the single output file is great for users and later for us to transform it into a PDF or?

Should we cache images? Say by default anything that was fetched should be cached locally and used in subsequent renders. Question is when to invalidate? Only on url change? On url change and a flag to disable the cache?

Local file support instead of a full url?
Such as /image.png, which looks for a image.png file within the current directory?
That would make it easy to manually store image files etc.

Thoughts?