Improve prelude-open-with

Now it uses `xdg-open` where it makes sense. It also accepts a prefix
argument that forces manual selection of a command to run.
give up on jump-char, ace-jump-mode is good for now
Don't try to save files when you don't have the permissions to do so
Quote buffer-file-name in prelude-open
Simplify prelude-rename-file-and-buffer
Merge https://github.com/bbatsov/prelude
add key-chords to jump to next =, ( and )