bug#25096: shr: Add a customizable face for links after being clicked

[Drew Adams <drew.adams@oracle.com>]

> Sorry for the confusion. I'm talking about it in eww.
> 
> Also, I'm not talking about a "visited-link" -- it is really about
> indication that a link was just successfully clicked. For example,
> in eww, visit an arbitrary page and move point to an arbitrary link and
> then press '&' (eww-browse-with-external-browser). There is no
> indication that the command has been executed. Even a dashed box
> lasting a few seconds on the clicked URL would help a lot.

So it's an EWW enhancement request.

An ephemeral box won't tell you later that you already visited
that URL in an external browser.

You could also have 3 faces for having used the link:

1. For having visited its destination in EWW.
2. For having visited its destination in an external browser.
3. For having visited its destination in both.

#1 could use a foreground color.  #2 could use an outline box.
#3 could use both.