Add missing config options, fix default values and layout

Hi, 
- I added two missing options (:crawl_finished_queue and :use_encoding_safe_process_job) to the list of Cobweb options because they're missing and I think they're quite important.
- I also clarified the default values for the two queue options: 1. they're strings, not class names and 2. they're different for sidekiq users.
- also added @s around the :valid_mime_types default value so it becomes properly visible on github (`*/*` on its own only emphasizes the slash). Might be reasonable to do this for all the symbols, but I didn't want to send a layout-only pull request.
Frank