wombag-search: Add w-search-open-in-wallabag

[EFLS]

I'm really liking Wombag as a Wallabag interface, especially because it feels so much like elfeed. I really love how you've made the live search so easy to use because of the help buffer.

While playing around with Wombag, I came up with the following function that might be useful to other users as well. I like to highlight passages as I read them, so I often visit articles in the web-interface. This pull request provides a function and a key for that.

Original commit comment: New function in wombag-search to open entry at point in the Wallabag web interface, using wombag-browse-url-function.