Feature request: Example code for custom keymaps

[hxlnt]

Is your feature request related to a problem? Please describe.
I can't for the life of me get custom keymaps working. I'm guessing it's not a bug and that I'm just doing something wrong because I'm new to Go, but some sample code would be super helpful!

Describe the solution you'd like
A sample in the examples folder that shows how to apply custom keymaps to a form, a group, and an individual input.

Describe alternatives you've considered
I didn't see any custom keymaps in the examples folder, nor did I see sample usage in the documentation.

[hxlnt]

Update: I did eventually get custom keymaps working when applied at the form level--something wasn't working at the individual field or group level. This might be related to #272.

I think I also wasn't aware that if you assign some custom keys, you need to assign all of them or else they're disabled. (Defaults aren't inherited if undefined.) Not sure if that's a bug or by design, but it wasn't apparent to me right away.

In any case, documentation would still be helpful on these fronts. Took a bit of debugging to track down the root causes. Thank you!!