Make log level configurable

[Indy2222]

Lowering the log level is very useful for debugging purposes.

This is blocked by #243.

[JackCrumpLeys]

Is this still valid? We have RUST_LOG now.

[Indy2222]

Is this still valid? We have RUST_LOG now.

You are right.

I am keeping it open until this is documented (single sentence + link to RUST_LOG docs would suffice).