Allow option of choosing sass/scss implementation

[TomCaserta]

What problem does this feature solve?

Since a couple versions back this plugin updated to use sass instead of node-sass. It would be good if the plugin could receive an implementation option for the scss compiler in order for the plugin user to specify what they would like to use.

For my use case I have many plugins (functions) to node-sass that are simply not yet possible with the sass implementation, this means I cannot upgrade to the latest version of this plugin.

This is similar to the option that is available in the sass-loader for webpack: https://github.com/webpack-contrib/sass-loader

What does the proposed API look like?

{
    // ...
    style: {
        preprocessOptions: {
            scss: {
                implementation: require('node-sass'),
            },
        },
    },
}

[znck]

Closing it as it is included in #300.