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.vim

@@ -1339,7 +1339,8 @@ let s:zen_settings = {
 \        },
 \        'empty_elements': 'area,base,basefont,br,col,frame,hr,img,input,isindex,link,meta,param,embed,keygen,command',
 \        'block_elements': 'address,applet,blockquote,button,center,dd,del,dir,div,dl,dt,fieldset,form,frameset,hr,iframe,ins,isindex,li,link,map,menu,noframes,noscript,object,ol,p,pre,script,table,tbody,td,tfoot,th,thead,tr,ul,h1,h2,h3,h4,h5,h6',
-\        'inline_elements': 'a,abbr,acronym,applet,b,basefont,bdo,big,br,button,cite,code,del,dfn,em,font,i,iframe,img,input,ins,kbd,label,map,object,q,s,samp,script,select,small,span,strike,strong,sub,sup,textarea,tt,u,var'
+\        'inline_elements': 'a,abbr,acronym,applet,b,basefont,bdo,big,br,button,cite,code,del,dfn,em,font,i,iframe,img,input,ins,kbd,label,map,object,q,s,samp,script,select,small,span,strike,strong,sub,sup,textarea,tt,u,var',
+\        'empty_element_suffix': ' />'
 \    },
 \    'xsl': {
 \        'extends': 'html',

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]