[Feature] Label only if unlabled

[swrobel]

I'd like to be able to apply labels only if none have been manually set by the author of the PR. If this is already possible, I'd love to know how!

[srvaroa]

The config itself should be simple, some property like append-only that skips issues with a label. The part I am not sure will work is how to distinguish who put the label in place. IIRC the GH API does not tell you who set a given label!