Add transform callback

[jakubpawlowicz]

See #842 (comment)

---

Here's one more idea for 4.0 - what if you could pass a callback as an option to transform any property? This would solve:

• rewriting / resolving URLs as one could update them by hand if nothing else works
• dropping properties if needed
• rewriting some values if needed

For example:

new CleanCSS({
  transformProperty: function (name, value) {
    if (name == 'background-image' && value.indexOf('invalid/path') > -1) {
      return value.replace('invalid/path', 'valid/path');
    }
  }
}).minify(...)

or

new CleanCSS({
  transformProperty: function (name, value) {
    if (name.indexOf('-o-') === 0) {
      return false; // drop a property
    }
  }
}).minify(...)

[jakubpawlowicz]

To be consistent with other level 1 optimizations it should rather be:

new CleanCSS({
  level: {
    1: {
      transform: function (propertName, propertyValue) {
        if (name == 'background-image' && value.indexOf('invalid/path') > -1) {
          return value.replace('invalid/path', 'valid/path');
        }
      }
    }
  }
}).minify(...)