Allow installation of Unity beta/alpha versions

[luiskrikey]

Not sure if this is even possible since I can't find those versions on the Unity download archive, but maybe someone can figure out where the official hub is grabbing its links from? Keep up the great work and thanks for creating this!

[r4dian]

You can see the source of links in the real hub's log file.

Or parse https://unity.com/releases/editor/beta/latest.xml