Strip tags from title

This is for issue #188

I also did some refactoring to make the sanitizer more flexible. Now it is in its own package and contains three public functions
- the html sanitizer/stripper function `Sanitize`
- text snipper to cut text into a specified length `SnipText`
- and the tag stripper function `StripTags` (this is a wrapper of `Sanitize`)

Previously the `Sanitize` function did three things (sanitize+strip+snip) so I think it's better to refactor it to several more specific functions.