* preparatory refactoring
* improved documentation markup
* ~ `enable_logging()` now also takes 2-parameters (environment variable name, default value);
~ `enable_tracing()` now also takes 2-parameters (environment variable name, default value);
* ~ `enable_logging()` now also takes 2-parameters (environment variable name(s), default value) as well as 1-parameter (is_enabled);
~ `enable_tracing()` now also takes 2-parameters (environment variable name(s), default value) as well as 1-parameter (is_enabled);