Motivation
----------
Current implementation does not allow to specify any formatting. It
even cannot output log level, because `writeln()` accepts
`varags[string]` and silently ignores everything before last argument.
Modification
------------
Perform formatting in the single place during substitution. Make log
level optional as all other substitution variables. Also make verbose
logging more informative and parseable.
Result
------
Correct handling formatting and substitutions. Machine-friendly default
output of verbose logger.
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