Add --base option to css compiler

[cblgh]

With a --base option (or similar naming) the following invocation:

go run compiler.go --css ./../../html/assets/css --html ./../../html --out ./../../html/assets/themes

could potentially turn into:

go run compiler.go --base ./../../html --css assets/css --out assets/themes

(inspired by skimming thru @Thomasorus's recent twitch stream :>