Programmatically select initial category

[Ryan McGill]

Hi! Love this library. It's clear you're taking great pains to keep performance fast and responsibilities clear. Allow me to bloat that with a feature request. 😉

I want to show users a specific, non-custom category when the element mounts. They should be free to pick any emoji they want, but I'd like to nudge them towards flags or food or animals depending on the task at hand.

I'm not familiar with supplying custom data sources so maybe there's already a way to do this... but even that seems a little heavy compared to (what appears to be) providing a custom currentGroupIndex when constructing the Picker.

Let me know if I'm missing something and/or I'm being unreasonable!