Bug: OSX-specific shortcuts don't work

[bryphe]

Some specific ones called out on Discord:

• cmd+shift+] (and [) to shift through open tabs
• cmd + , to open settings

We should definitely hook up these; as they don't conflict with Vim bindings, and should just work.

[mrvicadai]

Can I add a few other ones?

• Command + W: Close tab / buffer.
• Command + 1-8: Jump to the nth tab.
  Command + 9`: Jump to the last tab.

[yunti]

The change tabs shortcuts for macos now work. (I can't seem to find the commit where they got added.

[glennsl]

Thanks @yunti. Also Cmd+W should work. I've updated #1423 with Cmd+,, and will close this in favor of that.