Allow to use an id for custom categories

[Clebal]

When I add a custom emoji category, I cannot indicate a custom id, so a default one is used what is a bit uncomfortable when I have to fill the categories array.

Current case:

init({
    ...,
    custom: [{ name: "Basic", emojis: [ … ] }],
    categories: ["custom_1"]
})

After this change:

init({
    ...,
    custom: [{ id: "basic", name: "Basic", emojis: [ … ] }],
    categories: ["basic"]
})

[EtienneLem]

Totally makes sense, thank you!