Fix url filter behavior

[rodfersou]

Related to: #779

The result of the function should be inverted to keep in mind the same behavior when it is a boolean..
I mean when the function return false should not look the url.
This is my configuration in the end:

    {
      test: /\.scss$/,
      use: ExtractTextPlugin.extract({
        fallback: 'style-loader',
        use: [
          {
            loader: 'css-loader',
            options: {
              url: (url, resourcePath) => /\+\+resource\+\+/.test(url)
            }
          },
          'postcss-loader',
          'sass-loader'
        ]
      }),
    },

[alexander-akait]

@rodfersou thanks for issue, we can fix it only in next major release, i am afraid break loader for other developers

[rodfersou]

@evilebottnawi for me looks great!

[rodfersou]

🎉