Downloaded models are backed up to time-machine - un-necessarily

DiffusionBee downloads 9Gb of models. These are updated occasionally.

For users backing up their drives (this should be everyone!) this un-necessarily fills the backup with content that can be easily retrieved from the internet.

It's easy to prevent this in code in your app:

```objective-c
NSURL* url = //...
[[tildeDiffusionBeeDownloadsURL setResourceValue:@YES] forKey:NSURLIsExcludedFromBackupKey error:nil];
```

There's a way to do it on command-line if your app is mainly script based:

```sh 
tmutil addexclusion ~/.diffusionbee/downloads
```