Switch from swig to nunjucks

[jsit]

Swig is deprecated, should switch to nunjucks like default theme:

https://github.com/SassDoc/sassdoc-theme-default/pull/104/files

[pascalduez]

Hi @jsit,

Yes, that would be rad.

As you already noticed sassdoc-theme-blank was left a bit unattended and is lagging behind.
This is most likely due to the fact that we later worked on the theme generator. Sadly I think the generator itself is not in its best shape...

Thanks for your contributions!