bug#19556: eww: make URI rewriting fully customizable

[Eli Zaretskii <eliz@gnu.org>]

> From: Ivan Shmakov <ivan@siamics.net>
> Date: Sat, 10 Jan 2015 13:17:55 +0000
> 
> +(defcustom eww-uri-rewrite-hook
> +  '(eww-uri-file
> +    eww-uri-not-supported
> +    eww-uri-remote-prepend-http
> +    eww-uri-search)
> +  "List of functions called to deal with the argument to `eww'.
> +These functions will be called in order, with the argument given to
> +`eww' passed as their only argument, until one returns non-nil.  The
> +value returned will be used as the URI to fetch.
> +
> +Should no function of those listed return non-nil, `eww' will use its
> +argument unaltered."
> +  :version "25.1"
> +  :group 'eww
> +  :type 'hook
> +  :options '(eww-uri-file
> +	     eww-uri-not-supported
> +	     eww-uri-remote-prepend-http
> +	     eww-uri-search))

Why put the "standard" rules into the defcustom? why not leave them in
place as "plan B", and leave the hook for customizations only?  That's
what hooks are normally for -- _modifying_ the default behavior, not
supplanting it.  E.g., with your suggestion, what happens if someone
customizes the value to nil?

Thanks.