Auto-match annotations (feature)

[matt-deboer]

Feature Request: based on idea mentioned in #52 , it would be helpful if there was an auto-match setting for annotations such that, when a new annotation is added, a background process automatically compared the annotated image patch with other images in the dataset, and if the patch was "the same"(within some tolerance), the annotation would be automatically cloned to that image.

Note that this feature is only really useful for datasets created from video sequences.