Telemetry

I'd love me some opt-in telemetry sometime in the future.

Stuff like:
- Version
- Operating system
- Date of installation/first run

Giving us aggregate statistics like:
- Daily/weekly/monthly active installs
- Total install count
- Churn rate

We need to consider the possible privacy implications of collecting telemetry as well. I guess we'd need to make it opt-in but that'd leave us with some blind spots I'd rather not have, at least not this early in the projects life.

Telemetry might share some infrastructure with #114 (update checking).

An article regarding telemetry in open source software: https://alexgaynor.net/2015/sep/03/telemetry-for-open-source/