Add option to turn off specific CSSLint rules

This adds a config option to turn off specific CSSLint rules. For example I am currently using a CSS processor that includes `@import`ed files, which means that the `@import` warning is totally irrelevant for me. This change allows me to disable that specific warning, while still using CSSLint.

Example config to quit warning on `@import` statements in CSS:

```js
{
    "modes": {
        "css": {
            "commands": {
                "Tools:Check": {
                    "options": {
                        "import": false
                    }
                }
            }
        }
    }
}
```