diary: use :drop for :VimwikiMakeDiaryNote

I find this is one of the more common reasons I wind up with way too many
copies of the same file open.

This adds 2 new magic numbers to vimwiki#diary#make_note()'s second
parameter: 4 for `tab drop` and 5 for `drop`.  It also checks to make sure
:drop exists in the current build.

autoload/vimwiki/diary.vim

@@ -307,6 +307,13 @@ function! vimwiki#diary#make_note(wnum, ...) abort
       let cmd = 'split'
     elseif a:1 == 3
       let cmd = 'vsplit'
+    elseif a:1 == 4
+      let cmd = 'tab drop'
+    elseif a:1 == 5
+      let cmd = 'drop'
+      if exists(':drop')
+        let cmd = 'drop'
+      endif
     endif
   endif
   if a:0>1