Support Slackadays/clipboard

[Slackadays]

Instead of calling xsel and pbcopy for X11 and macOS respectively, you can use Clipboard to condense all those calls into one call to the CB binary. Additionally, CB's Wayland support would also fix https://github.com/janlelis/clipboard/issues/44 at the same time. You could also add a fallback option that uses a bundled CB binary if Clipboard Ruby can't find one installed on the system.

Edit: This would also enable support for the BSDs as well, since CB works on them. Double edit: This would also enable support for Linux/BSD where there isn't X11/Wayland because CB doesn't depend on them to exist.