add: Added .keep command which will add the current guild to the list of keptguilds. This is needed for the future database purge.

src/NadekoBot/data/strings/commands/commands.en-US.yml

@@ -4598,4 +4598,11 @@ afk:
     - ''
   params:
     - msg:
-        desc: "The message to send when someone pings you."
+        desc: "The message to send when someone pings you."
+keep:
+  desc: |-
+    The current serve, won't be deleted from Nadeko's database during the purge.
+  ex:
+    - ''
+  params:
+    - {}