create labels for versions

[embear]

Having a visual undotree is really useful. Many thanks for that!

Identifying a specific version still is hard. A really nice addition would be the ability to add a label to a certain version. This would make it possible to mark a specific version as some kind of "snapshot" for example as "last known working version".

I know that the undo feature of vim is not a VCS, but I think it would be nice to have such a feature.