Add scss to template engines

[fakefarm]

Per https://github.com/middleman/middleman/issues/2101#issuecomment-481890942

I added a new section, doing my best to explain my specific situation in general terms. I am happy to edit, or remove it.

[tdreyno]

This is great @davewoodall, thank you. I may expand a little, as it applies to any partials (erb, haml, etc), but this is a great start.

[lolaodelola]

Hey folks, is this change gonna be merged in anytime soon? I went on a bit of a wild goose chase before I came across this & it'd be great if it was already in the docs 😊

[tdreyno]

Looks like there's a conflict on the branch