update delimiters as fragment
make handler of setting attributes of element
detect invalid URL

* remove |kPref.showSpecialSchemes| and always show all entries
* rename |kUI.special| to |kUI.unknown| to handle a special scheme and also an
  invalid URL
* set the style for an unknown URL to 'color:red' to warn strongly, and change
  the style for same page URL to 'font-style:italic'