feat(prettyprint): improve pretty-print directive to work with unescaped html

BREAKING CHANGE: w11k-pretty-print directive now has to be placed at the element containing the code

Example:
<pre w11k-pretty-print lang="html">
<div class="screen">
  &lt;div w11k-slides&gt;&lt;/div&gt;
</div>
</pre>
docs(demo): adjust w11k-pretty-print usage example to improved directive
chore(project): bump version to 0.4.0
docs(changelog): update changelog
Merge branch 'release/0.4.0' into develop