Allow choice between GitHub releases

This PR allows users to choose which asset to download in the event that there are multiple compiled assets available (as seen [here](https://github.com/rbray89/ActiveTextureManagement/releases)) asking them which to use when adding/updating instead of simply using the first.
nbd, but in Java 7, you can omit the second set of generics.
```
HashMap<String, Element> released = new HashMap<>();
```