Reline::Face modifications

[rubyFeedback]

Today I found this snippet:

# configure autocomplete dialog colors
if defined? Reline::Face
  Reline::Face.config(:completion_dialog) do |conf|
    conf.define(:default, foreground: "#cad3f5", background: "#363a4f")
    conf.define(:enhanced, foreground: "#cad3f5", background: "#5b6078")
    conf.define(:scrollbar, foreground: "#c6a0f6", background: "#181926")
  end
else
  IRB.conf[:USE_AUTOCOMPLETE] = false
end

While this is kind of specific to IRB, perhaps Reline::Face can also be used
outside of IRB, so we may want to document this or at the least provide
a simple standalone example for it (in the repository here).

The original source is from here:

https://dev.to/joshdevhub/configuring-irb-autocomplete-3n4d

I do not know whether the author would be ok if we put a standalone example
into reline, but I assume he may be ok; even if not we can probably add our
own example that is similar. (Right now I actually don't know how to use
Reline::Face outside of IRB. That's another thing we should eventually document,
it seems reline is catering primarily to IRB but the use case is I think more generic,
as we can use reline for simple user-input too, in any REPL.)