Add option to `no-unassigned-import` for allowed glob

When using the style and css webpack loaders, sometimes I want to be able to import a CSS file which will result in injecting the styles in the DOM (a side-effect) but this is acceptable in some use cases (I realize there are CSS modules, but importing things like bootstrap etc.)

So I suggest:

```javascript
"import/no-unassigned-import": [2, {
  "allowed": ["*.css"]
}]
```

Thoughts?