CSS4 custom properties

## Will CSSlint be supporting CSS4 in the future?

getting  an error when linting custom CSS properties.
### Error message:

```
 _Expected RBRACE at line 47, col 5_
```
### CSS snippet

```
:root {
      --yellow-tan-gradient: linear-gradient(45deg, rgba(247,242,171,1) 0%, rgba(254,210,138,1) 49%,    rgba(254,210,138,1) 100%);
}
```

It doesn't like the two "--" in front of the name
