Ops: Inject SASS variables and imports into build process

[xak]

We have the need to build different versions of the library like a dark theme and IE 11 compatible version with CSS var fallbacks.

It would be great to automate this ...

Dark theme

Build all.scss with @import "./theme/dark";

CSS vars

Build all.scss with $fd-support-css-var-fallback: true;

Currently this has to be done either manually or with a new master, e.g., all file for each configuration.

See https://stackoverflow.com/questions/31148803/injecting-variables-during-sass-compilation-with-node