Setup custom key bindings for windows/linux

As you documented, we (windows/linux users) have to define our own key bindings to navigate through panes.

> When maximized, you can navigate to your other panes with: <kbd>cmd+ctrl+→</kbd> or <kbd>cmd+ctrl+←</kbd>

> Windows / Linux users will need to setup their own key bindings

But why do we have to? <kbd>ctr+alt+...</kbd> is not used by windows nor st2. Or are there any other thoughts I've missed?

```
[
    { "keys": ["ctrl+alt+enter"], "command": "max_pane" },
    { "keys": ["ctrl+alt+right"], "command": "shift_pane" },
    { "keys": ["ctrl+alt+left"], "command": "unshift_pane" }
]
```