Add development/rebol

[YellowApple]

Upstream: http://rebolsource.net/

Rebol is apparently self-hosting, which means one would need to download a binary release. TBD if I want to leave it at that or actually use it to build Rebol specifically for Slackware (the latter is probably going to end up being "preferred", but I ain't sure if it's worth the extra management overhead here).