Support logging via RUST_LOG

[alexheretic]

• Support logging enabled when stderr is not a terminal or by setting env var RUST_LOG. E.g:
  • RUST_LOG=ab_av1=info "info" level logs various progress results like sample encode info
  • RUST_LOG=ab_av1=debug "debug" level logs include ffmpeg calls
• Fix sample-encode printing output to non-terminals.
• Omit "Encode with: ..." stderr hint for non-terminals.

Resolves #204 Relates to #58