[FEATURE] Allow Keyboard Remapping for In-App Navigation

Hi,

I've tried cursive for several days, however, I couldn't find the documentation for remapping a keyboard input to in-app action like navigation.
It would be great if it's possible to remap/rebind the keyboard input to some in-app action, for example:

```rust
let cs = cursive::Cursive();

cs.add_global_callback('k', |c| c.event.navigation.up());
/* or */
cs.add_global_keymap('h', cursive::event::Key::Left);

//...
```

Kindly let me know if it's already possible or if I'm missing something.