Improved rate limiting

[rusq]

Fixes #12

• Separate limiters for files, conversation and threads, and retries on 429 for all of those methods.
• Generic withRetry function that retries upon receiving slack.RateLimitedError, waiting the RetriedAfter duration;

Minor:

• switch to dlog library for logging
• Introduce ability to trace the execution using runtime/trace
• build version
• allow the secrets to be stored in .env, or .env.txt or secrets.txt for convenience of notepad users.

[rusq]

I don't know what happened here looks like it was brutally reformatted, so that it's own mother does not recognise it.