Add ability to provide a cluster function in kubecontext segment

This makes it possible to customize the displayed cluster name, e.g.
`gke_project-name-1337_europe-west1-c_production` -> `production`

EXTENDED_CONFIGURATION.md

@@ -43,6 +43,19 @@ another
 You can pick up a fix for the latter from
 [a fork of zsh](https://github.com/romkatv/zsh/tree/gentle-reset-prompt).
 
+`POWERLEVEL9K_KUBECONTEXT_CONTEXT_FUNCTION (FUNCTION) [default=""]`
+
+Add your own function to modify the k8s context name, e.g. remove superfluous information:
+
+```bash
+# Input: gke_project-name-1337_europe-west1-c_production/ns
+# Output: production
+function k8s_context_short() {
+  echo "$1" | sed -E 's/.*-[a-z]_(.*)\/.*/\1/'
+}
+POWERLEVEL9K_KUBECONTEXT_CONTEXT_FUNCTION="k8s_context_short"
+```
+
 When using gitstatus, there is an extra state called `LOADING` that is used by `vcs` prompt
 segment when it's waiting for git status in the background. You can define styling for this
 state the same way as for the other states -- `CLEAN`, `UNTRACKED` and `MODIFIED`. You can

internal/p10k.zsh

@@ -2126,6 +2126,9 @@ prompt_kubecontext() {
         fi
       done
     fi
+    if [[ -n $POWERLEVEL9K_KUBECONTEXT_CONTEXT_FUNCTION ]]; then
+      ctx=$($POWERLEVEL9K_KUBECONTEXT_CONTEXT_FUNCTION $ctx)
+    fi
     _p9k_cache_set "$ctx" "$suf"
   fi