Add color viewing convenience?

Recently, when choosing various colors for stuff, I've wanted a better way to display and compare colors. I created a simple script/function for this:

```fish
function swatch
set -l height_in_lines 2

set -l width_in_chars (math --scale=0 $height_in_lines x 2.5) # 2.5 = ratio

for i in (seq $height_in_lines)
for arg in $argv
set_color -b $arg
string repeat --no-newline --count $width_in_chars ' '
set_color normal
end
echo
end
end
```

It looks like this:


I was wondering if Fish might be interested in adding something like this. Perhaps as a subcommand to `fish_config`, perhaps as an option to `set_color`, e.g. `set_color --swatch`?

I think this is a pretty useful tool for interacting with `set_color` 😄

I'd also be happy to implement this, as it shouldn't be too difficult in C++ or Fish :)