Improve auto placeholder feature

autoload/neosnippet/parser.vim

@@ -238,6 +238,14 @@ endfunction"}}}
 
 function! neosnippet#parser#_initialize_snippet(dict, path, line, pattern, name) "{{{
   let a:dict.word = substitute(a:dict.word, '\n\+$', '', '')
+  if a:dict.word !~ '\n'
+        \ && a:dict.word !~
+        \    neosnippet#get_placeholder_marker_substitute_pattern().'$'
+        \ && a:dict.word !~
+        \    neosnippet#get_placeholder_marker_substitute_zero_pattern()
+    " Add placeholder.
+    let a:dict.word .= '${0}'
+  endif
 
   if !has_key(a:dict, 'abbr') || a:dict.abbr == ''
     " Set default abbr.