add fallback template for HTML tags

[peterstadler]

This PR fixes #193 by introducing a fallback template for arbitrary HTML tags.
Unknown HTML tags will be stripped off which needs to be documented/warned about in the FAQ.

[peterprovaznik]

Looks very good to me!

Edit: Sorry, when checking with the image just to be sure, I noticed that the content that used to be in for example a <h2> tag is now a direct child of the <annot> element... are we ok with that?

<annot>
  ...
  <p>
    <rend fontstyle="italic">Please note: The content of this file has been modified for demonstration purposes and does not represent accurate real-world data.</rend>
    </p>
     add fallback template for HTML tags
    <rend>#196</rend>
  </annot>

[peterstadler]

I agree that the result is not "great" in terms of MEI (or text structure in general) but there seems to be no easy solution to filter out all possible (eventually nested) HTML elements and wrap them appropriately.

My proposal would be to merge this PR since it fixes the initial bug of destroyed TinyMCE editor and look after improving the output in a later step.