feat: Support encoding with arbitrary charsets

[taion]

This is just a GitHub editor spike, but I wanted to see what you thought about this. I don't see a generic "base X" library for Python analogous to e.g. base-x for JavaScript, but it looked like it would be pretty easy to adapt this library to do that.

My proximate motivation here is just that base36 doesn't take bytes, so the API is a bit annoying to use.