Add label after closing an issue

[OllisGit]

This PR is related to this issue #298

Now you can the following attribute to the stale.yml file and after an issue is closed by the bot an label is added.

closedLabel: "closedByBot"

I the named label doesn't exists, it is created.

You can disable the label assignment by switching the config to false.

closedLabel: false

[danielb987]

Can this PR be merged? We would like to use it in the JMRI project.