Sass filter: Make sass use the webassets cache if possible.
Sass filter: Make sass use the webassets cache if possible.

Relates to #40, #67.

aconrad/webassets,JDeuce/webassets,glorpen/webassets,john2x/webassets,JDeuce/webassets,scorphus/webassets,glorpen/webassets,john2x/webassets,heynemann/webassets,heynemann/webassets,florianjacob/webassets,aconrad/webassets,wijerasa/webassets,glorpen/webassets,scorphus/webassets,aconrad/webassets,heynemann/webassets,wijerasa/webassets,florianjacob/webassets,0x1997/webassets,0x1997/webassets