Add option to disregard labels in dataset

[alanakbik]

Sometimes, we may not want to train a classifier over all labels in a dataset. For instance, we might want to learn an NER tagger for all organization names only and disregard all other labels.

This issue tracks the progress of adding this feature to Flair.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.