adds support for vertically aligning html content within a layered <div>

when the value of the option `positioned_element_class` is an array of strings:

* if the <area> element corresponding to the <a> element being constructed
  contains the following two attributes:
    - "x-" + classname
    - "x-" + classname + "-valign"

* if the value of the "valign" attribute is "middle"

then the html content is wrapped in additional html,
which renders the original html content vertically centered within the <a>

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>"
         x-caption-text-valign="middle" />