Exclude inline data URI

[attiks]

We're using sass and we inline most of our background svg's using data URI's, but we need a way to exclude them before inlining the critical css. For the moment we run sed from inside our Gruntfile (sed -i '/data:/d' css/critical.css), but it would be easier if it was handled by criticalCSS.

[jefflembeck]

We have a forceInclude, I think a forceExclude is a reasonable option.