feat: add rewrite option

[tivac]

So that the postcss-url functionality is configurable and can be disabled in a less surprising way.

// postcss-url disabled
var processor = new Processor({ rewrite : false });

// Passing options to postcss-url
var processor = new Processor({
    rewrite : {
        url : "inline"
    }
});

Still needed:

• Docs
• Expose as CLI flag so rewriting can be disabled from CLI
• Changelog entry

⚠️ This will require a major version bump, because passing after : [] will no longer prevent postcss-url from running ⚠️