Allow for specifying a log-level when initializing a logger

[karalekas]

Kept track of via a new parameter:

(defparameter *log-level* nil)
(setf (documentation *log-level* 'variable)
      "A non-negative INTEGER that describes what level of logging to perform.
       Entries by default are given a log-level of 0, akin to DEBUG logging.
       Larger integers are used for logging more critical events.")

Set by providing a :log-level argument to the with-transient-logger macro:

(with-transient-logger (:log-level 1) ...)

Also adds some frills to print-log.

[ecpeterson]

We chatted a bit out-of-band about something other than integers for log levels, but that can be left for another day.