Capture TTY as custom column

I'd like to capture the tty as a custom column but I cannot work out the best way to achieve this.

I get `not a tty` if I try to specify

```
hishtory config-add custom-columns tty tty
```

and I cannot work out how to get this syntax to work without either getting `TTY` or an empty string...

```
hishtory config-add custom-columns tty 'echo \$TTY'
```
