add option for customizing the empty tag suffix

Example: To expand the br tag as <br/> instead of the default <br />, put this
in your .vimrc:

let g:user_zen_settings = {
\  'html': {
\    'empty_element_suffix': '/>',
\  },
\}

autoload/zencoding/lang/html.vim

@@ -335,7 +335,7 @@ function! zencoding#lang#html#toString(settings, current, type, inline, filters,
     let str = "<!-- " . comment . " -->\n" . str
   endif
   if stridx(','.settings.html.empty_elements.',', ','.current_name.',') != -1
-    let str .= " />"
+    let str .= settings.html.empty_element_suffix
   else
     let str .= ">"
     let text = current.value[1:-2]