Strip tags from post content

I think it makes sense to remove all tags from post_content.

This has the following advantages:
* Smaller index
* No search results for "technical" terms such as `html`, `script`, `media`, `utf8` etc.
* When using highlights, the returned excerpts don't contain part of html tags. Currenlty, it might return sth. like `ndex.html" class="button">Some text here with the search term in it ...`.