feat: allowedVersions

A semver range defining allowed versions for dependencies

| name | value  |
| ---- | ------ |
| type | string |

Use this - usually within a packageRule - to limit how far to upgrade a dependency. For example, if you wish to upgrade to angular v1.5 but not to `angular` v1.6 or higher, you could defined this to be `<= 1.5` or `< 1.6.0`:

```
  "packageRules": [{
    "packageNames": ["angular"],
    "allowedVersions": "<=1.5"
  }]
```