Better support progressive enhancement in Conflict Validator and Conflict Resolver

See: https://github.com/css-blocks/css-blocks/pull/62#discussion_r167104549

After #62 lands we should not have problems with uncaught conflicts. However, there is room for improvement with how css-blocks handles progressive enhancement and vendor prefixed property resolutions. Ex:

```css
block-a.foo {
-webkit-transform: translateY(50%);
transform: translateY(50%);
}

block-b.foo {
-webkit-transform: translateX(50%);
transform: translateX(50%);
-webkit-transform: resolve('block-a.foo;'); /* Do we need this resolution if we're PA aware? */
transform: resolve('block-a.foo;');
}
```

Start by reading the discussion on PR #62 and the progressive-enhancement example in this issue. Define the expected handling of vendor-prefixed properties and whether matching resolve declarations remain necessary; done means the Conflict Validator and Conflict Resolver handle these cases without uncaught conflicts.

Written by the indexing model from the issue text.