Add default \es keybinding for "prepend sudo"

[gpanders]

Description

This creates a new function __fish_prepend_sudo and a new default keybinding Alt+S that prepends sudo to the beginning of your current commandline.

Example:

$ vim /etc/hosts

pressing Alt+S yields

$ sudo vim /etc/hosts

This does not move the cursor.

This was inspired by the comment here.

Personally, I think this might allay some of the agitation around the lack of the !! operator in fish, since sudo !! can now effectively be done with Ctrl+p Alt+s, which is even easier!

[krobelus]

One possible issue here is that sudo might not be available on all systems fish supports (unlike say man)

[faho]

Merged, thanks!