Added the option to open vimwiki in a pane.

1       tabedit
2       split
3       vsplit

Similar to opening a tab,::

vimwiki#base#goto_index(v:count1, 1)

we can now open a split with::

vimwiki#base#goto_index(v:count1, 2)

and a vertical split with::

vimwiki#base#goto_index(v:count1, 3)

autoload/vimwiki/base.vim

@@ -1325,8 +1325,12 @@ function! vimwiki#base#goto_index(wnum, ...) "{{{
     let idx = 0
   endif
 
-  if a:0
+  if a:0 == 1
     let cmd = 'tabedit'
+  elseif a:0 == 2
+    let cmd = 'sp'
+  elseif a:0 == 3
+    let cmd = 'vsp'
   else
     let cmd = 'edit'
   endif