Logging

[adamnfish]

The logger should be off by default, and on (to stdout/err) when --debug is provided. Sadly slf4j (and thus scala-logging) does not support setting the log level at runtime.

TODO: Work out the incantations required to obtain the underlying logback-classic logger and set its log level based on the presence (/absence) of a --debug flag.

Related: Scatter logging all through the application.
Related: The --debug flag could optionally take a level if we want to take full advantage of the logger's power.