Add labels for priorities of issues (exercism#416)

Author: mpizenberg

.appends/.github/labels.yml

@@ -14,3 +14,11 @@
 - name: "wontfix"
   description: ""
   color: "ffffff"
+
+- name: "x:priority/high"
+  description: "Has a higher priority than other issues"
+  color: "8FDAF8"
+
+- name: "x:priority/low"
+  description: "Has a lower priority than other issues"
+  color: "8FDAF9"