Exclusive label groups

[jaywonchung]

Sometimes, you want to express reading progress with labels, and a paper can only have exactly one of three states: ['queued', 'active', 'done'].

Without exclusive label groups, you would have to run commands in this order:

$ ls zeus | set is queued  # You made up your mind to read this paper soon.
$ ls zeus | set not queued | set is active
$ ls zeus | set not active | set is done

Having to run set not queued and set not active are a bit annoying.

Exclusive labels groups can be defined in the configuration group like the following:

[output]
exclusive_labels = [ ['queued', 'active', 'done'] ]

and a paper can only have zero or one of the labels part of the same exclusive label group. set-ing the second label will delete and override the first label.

[jaywonchung]

Implemented in 351ebb7.