Add feature to filter issues by excluding labels #1507

When I was looking for issues I could takle I wanted to filter all that included the "help wanted" but not the "has PR" label.

A query for that would look like this: label:"help wanted" -label:"has PR"

The Label filter drop-down only allows to filter issues that include one or more labels.

I would love to have the possibility to select (or even toggle) a label to be excluded by pressing some key (maybe alt ?). If this would be implemented there would be the need to indicate that that label in excluded instead of included in the filter drop-down, because right now it is still checked (which is more of a bug on Github's site I already encountered in #1363).

Also see: https://github.com/isaacs/github/issues/65