adds support for displaying multiple layered <divs>

changes the option `css_class` to: `positioned_element_class`

  * when the value is a string, it behaves the same as before

  * when the value is an array of strings:
    <a style="position:absolute; width:Apx; height:Bpx; left:Cpx; top:Dpx;">
      <div class="PEC[0]" style="width:Apx; height:Bpx;"></div>
      <div class="PEC[1]" style="width:Apx; height:Bpx;"></div>
      ...
      <div class="PEC[N]" style="width:Apx; height:Bpx;"></div>
    </a>

  * when the value is an array of strings:
    if the <area> element corresponding to the <a> element being constructed
    contains an attribute that matches one of these strings
    (prefixed with: "x-"), then the value of the attribute
    is added to the corresponding <div> element as html content.

example:
  * option value:
      "positioned_element_class" = ["background-overlay","caption-text"]
  * html:
      <area shape="rect" coords="0,0,100,100" href="#example"
         class="onhover odd not-even"
         x-caption-text="<span>hello world</span>" />