Allow extensions on Nunjucks

[fserb]

Extensions on Nunjucks can't be passed by regular options, they need to be added by addExtension, so we add a new extensions option and register them.

I had to move it inside the passed options, because options.extensions is already a thing. We could do options.nunjucks_extensions if you want it to be outside options.options. WDYT?

[oscarotero]

Great, thanks for this.
For naming consistency with postcss and markdown, I'd use plugins as the name for this option.

[fserb]

Done. Also moved to top level options, as the other plugins do.

[oscarotero]

Thanks! 👍