Support custom node-sass importer compileOption.

[jgerigmeyer]

Right now, any custom importer fn is ignored (replaced by the sass-extract specific importer), and not passed through to node-sass. I see this issue has been mentioned in both sass-extract loader (jgranstrom/sass-extract-loader#1) and babel-plugin-sass-extract (jgranstrom/babel-plugin-sass-extract#1), but I don't see a fix for it yet.

For example, I have sass files that use webpack's ~ shortcut to import node modules:

@import '~my-module/sass/utility';

When not using webpack to compile, I pass a simple importer option through to node-sass:

importer: url => {
  if (url[0] === '~') {
    url = path.resolve('node_modules', url.substr(1));
  }
  return { file: url };
},

This doesn't work with sass-extract, however (see https://github.com/jgranstrom/sass-extract/blob/master/src/extract.js#L39).

I'll submit a PR shortly that fixes this.