* Support "read more" for Activity-Summary
* add support for excerpt
* use the same function also for the shortcode
* add mentions, hashtags and links support to `the_excerpt`
* do not make unnecessary extra db calls
* fall back to default if options is empty
* remove unused hook
* some fixes
and improvements based on the feedback of @MatzeKitt
* add phpdoc
* also extract hashtags from excerpt
* use filter, so that it is possible to also support mentions
and maybe tags in the future
* Add PHPDoc
* simplify code
* fix test
