WIP: Allow log level to be set contextually

You can specific the log level for a specific class, or any package
(which will automatically include subpackages too).

For example, you could debug service loading with:

  System.setProperty(
    "scijava.log.level.org.scijava.service.ServiceHelper", "debug");

There are probably performance implications with this implementation,
though. As soon as *any* custom log levels are set, *every* getLevel()
call triggers some substantial string manipulation logic.