Add command to prune (upstream gone).

Custom-Commands-Compendium.md

@@ -147,6 +147,20 @@ customCommands:
     description: "prune local branches that have been merged to master"
 ```
 
+## Pruning branches no longer on the remote.
+
+Implementation inspiration [here on Stackoverflow](https://stackoverflow.com/a/33548037/2081835). Basically, delete all the branches displaying `(upstream gone)`.
+
+```yml
+customCommands:
+  - key: "G"
+    command: |
+      git fetch -p && for branch in $(git for-each-ref --format '%(refname) %(upstream:track)' refs/heads | awk '$2 == "[gone]" {sub("refs/heads/", "", $1); print $1}'); do git branch -D $branch; done
+    context: "localBranches"
+    description: "Prune local branches no longer on its remote; (G)one."
+    loadingText: "Pruning gone..."
+```
+
 ## committing via [Commitizen](https://commitizen-tools.github.io/commitizen/) (`cz c`)
 
 ```yml