Diagnosticism

* ~ fixed recently introduced defect in ``enable_tracing()``

* + added ``set_log_filter()``, which takes a severity-level as a threshold or a dictionary that controls emission of individual severity-levels

* + added more documentation and parameter assertions

* ~ renamed **examples/flog_fail.(go|md)** => **examples/logf_fail.(go|md)**

* ~ wholesale rename of ``diagnosticism`` => ``Diagnosticism``

* + added ``warn()`` (as an analogue of Ruby's ``Kernelwarn()``)
* + added ``conrep.set_default_usage_prompt()`` to allow a default usage prompt for ``abort()``
* ~ general improvements to documentation