Add `browsers` option

* Accept same value as Autoprefixer and override it
* Override many options in one go
* Based on CanIUse database (exactly as Autoprefixer)

For example, if you set:

```javascript
{
  "browsers": ["ie 9"]
}
```

You will automatically have:

```javascript
{
  "autoprefixer": { "browsers": ["ie 9"] },
  "rem": false,
  "opacity": false,
  "pseudoElements": false
}
```

(fix #25)