Iconfont attempts to resolve in mixin's body
Hi,
thanks for an awesome plugin!
I am trying to use plugin with scss mixins and it seems it tries to resolve `font-icon` on mixin level.

example of code:
```
.custom- icon {

  @mixin icon($name) {
    &:before {
      font-icon: url("./images/#{$name}.svg") !important;
    }
  }

  &-paper-plane { @include icon("paper-plane") }
}
```

results in: 
`index.js:16 Error: Module build failed: Error: Can't resolve './images/#{$name}.svg'`

webpack styles part:
```
       test: /\.(css|scss)$/,
        use: [
          'style-loader',
          {
            loader: 'css-loader',
            options: { sourceMap: true }
          },
          {
            loader: 'sass-loader',
            options: { sourceMap: true }
          },
          {
            loader: 'postcss-loader',
            options: {
              plugins: (loader) => [ new IconfontWebpackPlugin(loader) ]
            }
          }
        ]
```

Am I doing something wrong or this is an expected behavior?