zk: Quicknote alias defaults to current date

When creating a quick note with `nnn` on the command line, instead of
creating an 'untitled note' if no title is passed, we create a note that
is named after its creation date.

writing/zk/config/sh/alias.d/zk.sh

@@ -27,7 +27,7 @@ if [ -n "${WIKIROOT}" ]; then
         _zk_wiki new "$@"
     }
     nnn() { # 'new quicknote'
-        _zk_wiki new -t "${*}" inbox
+        _zk_wiki new -t "${*:-$(date)}" inbox
     }
     nnl() { # 'new note log'
         _zk_wiki log "$@"