Register handler for Gemini:// links

[jmthackett]

I'd like to be able to tap-through rather than copypaste to use Gemini links. This seems to be possible on android in the way described below:

https://stackoverflow.com/questions/2448213/how-to-implement-my-very-own-uri-scheme-on-android#2448531

I'm happy to open a PR if you'd find it helpful, although it looks pretty small so it might be quicker for me not to. Up to you - just let me know.

Cheers for the lovely software!