fix(highlight): use <div> when wrap is disabled

[curbengh]

• BREAKING CHANGE: <figcaption> is replaced with <div> when wrap is disabled
• feat(prism): caption #227 (comment)

Affects users of following config:

# _config.yml
highlight:
  line_number: false
  wrap: false

# or
prismjs:
  enable: true

New rendered html:

<pre>
  <div class="caption">
    caption here
  </div>
  <code>codeblock</code>
<pre>

suggested css:

pre div.caption {
  font-size: 0.9em;
  color: #888;
}

pre div.caption a {
  float: right;
}

[coveralls]

Coverage increased (+0.008%) to 97.436% when pulling 23372e0 on curbengh:wrap-caption into bf42b66 on hexojs:master.