extract-svg-sprite-webpack-plugin: limit file scope

[patric-eberle]

In our case it would be handy if the scope of svg files, that will be bundled into the sprite could be limited by configuration. Unfortunately some browsers don't support references inside the svg file itself (e.g. applying a gradient background). Such files should be kept separately.

See https://stackoverflow.com/questions/44235845/svg-use-and-gradients-as-fill for visualization of the problem.