Binding keys to tmux program commands fails silently

I'm trying to migrate my existing iTerm2 workflow over to Alacritty + tmux. I would like to preserve high level macOS commands like Command + 1 to select the tmux "tab".

As you can see below, I can confirm that Alacritty is sending the `say` command ok. However, I can't make it work with `tmux` commands. I have confirmed that I can successfully send tmux commands from another terminal emulator -- so I'm assuming that Alacritty is dropping the ball somewhere.

```yaml
key_bindings:
# This works as expected
- { key: Key3, mods: Command, command: { program: "say", args: ["one", "two", "buckle", "my", "shoe"] } }
- { key: Key4, mods: Command, command: { program: "say", args: ["'one two buckle my shoe'"] } }

# This does not appear to send tmux commands
- { key: Key1, mods: Command, command: { program: "tmux", args: ["select-window", "-t", "1"] } }
- { key: Key2, mods: Command, command: { program: "tmux", args: ["select-window", "-t", "2"] } }

# Neither to these alternative formats:
- { key: Key1, mods: Command, command: { program: "tmux select-window -t 1", args: [] }}
- { key: Key2, mods: Command, command: { program: "tmux", args: ["select-window -t 2"] }}
```

macOS 10.12.6
alacritty v0.1.0

BTW, is there a way I can debug this sort of thing? I'm not finding anything in the system console logs.