controlling logback configuration in standalone app

Application end-users must have the ability to change the logback logging level applied to all of the DK code, without having to explode the application jar.

Logback looks for a config file named logback.xml on the classpath. So first we copy a new logback.xml file into cwd and try setting the classpath on the command line:

java -cp . -jar diffkit-app.jar

No joy.

But this works: 

java -Dlogback.configurationFile=./logback.xml -jar diffkit-app.jar

Simple enough.

Also, logback can find the logback.xml file embedded in the application jar if the System property is defined this way:

java -Dlogback.configurationFile=conf/logback.xml -jar diffkit-app.jar

In this case, logback treats the property value as a classpath resource specification.