Allow customizing the pane formatting

The pane list was a bit too verbose for me, so I made it customizable. I'm using the following format for now:

```
  let g:slimux_pane_format = '[#{pane_current_command}: #{pane_title}] #{?pane_active,(active),}'
```

Maybe `pane_current_command` could be included by default, since it makes it very easy to identify REPLs.