add optional unofficial releases watch list

[unitycoder]

To get alpha versions listed earlier, can search them with google api. (sometimes they are available, like 6000.1.0a4 is now, before its listed in releases api)

• [ ] Add settings option for "fetch unofficial releases list" or so
• [ ] Fetch data from my gist https://gist.github.com/unitycoder/dbe6b0959162bccd099976d13bcb3c0f
• [x] Setup script to update that gist automatically (from my server)
• [ ] to make it more transparent, when user clicks install buttons, could show download url (can also validate that its unity3d domain)
• [ ] add 6000.0.x releases also?
• [x] dont display releases that are already in the unity releases list https://services.api.unity.com/unity/editor/release/v1/releases?limit=20&offset=0