Update extract text plugin version [WebPack2]

Hi,

In extract-svg-plugin.js, the syntaxe of extract-text-webpack-plugin is deprecated : this.extractPlugin = new ExtractPlugin(this.id, this.filename, this.options);

should be something like : this.extractPlugin = new ExtractPlugin({ filename: this.filename, id: this.id});

There might be other changes as it’s not enough to make this work again.

Thanks a lot.

[farwayer]

@kisenka I made sample repo with test:

git clone https://github.com/farwayer/svg-sprite-loader-bug
cd svg-sprite-loader-bug
npm install
npm start

[farwayer]

@kisenka Thanks for fixing. Now extracting working well. But one problem still exists. ExtractSVGPlugin outputs [object Object] as id.