Changed completion of history an bookmarks (#27).

Now we distinct between history completion and bookmark completion.

1. Bookmark completion is triggered by ':open !<tab>' to complete all
   bookmarks or by ':open !tag1 tag2' to complete only those bookmarks that
   have tags with the prefixes tag1 and tag2.
2. History completion is changed to split the search query at the space char
   and to search for all given words in the history. Only those history items
   are completed, that match all given words or tags.
3. Search completion works now also tag based like the url and bookmark
   completions.