better documention for timestamp-lines

I needed to add: timestamp-lines=true to: buildkite-agent.cfg in order to get the timestamps to show up in the logs.

It would be nice if this option was added to buildkite-agent.cfg but commented out with a description above like several of the other config options.

It would be even better if this could be dynamically enabled and disabled by setting the BUILDKITE_TIMESTAMP_LINES env variable. This way users don't need to deal with updating the cfg file and restarting the agent. Setting that env variable does not seem to work without first editing the buildkite-agent.cfg.

(originally https://github.com/buildkite/feedback/issues/349)

Start with buildkite-agent.cfg and the existing handling of the timestamp-lines option, then trace how the BUILDKITE_TIMESTAMP_LINES environment variable is read. Done means the option is documented in the configuration and the environment variable can enable or disable timestamp lines without requiring a configuration-file edit and agent restart.

Written by the indexing model from the issue text.