Opt-in telemetry

[nikomatsakis]

We are going to need to iteratively improve. We should allow users to opt-in to telemetry and then gather a record of the conversations, bits of source, as well as their +1 / -1 (or even more detailed...) feedback. In my ideal world it would be trivial to enable telemetry ONLY on open-source projects (as indicated perhaps by the license in the Cargo.toml).