Added single attributed attr.

[ibnuda]

Like "disabled". For example:

let single = SingleAttr
let fieldset = elem "fieldset"
let input = elem "input"
let legend = elem "legend"
fieldset [ legend [ text "Rename Folder" ]
           input [ "class" %= ""
                   "name" %= "olddirname"
                   "value" %= "/"
                   single "disabled" ]

Which generates

<fieldset>
  <legend>Rename Folder</legend>
  <input class="" name="olddirname" value="/" disabled/>
</fieldset>