Re : How to build statistics for an update run

I have pushed a PR for supporting setting a ChangeExecListener and Properties file for the listener instance  via the command line. 

The properties added are:
* changeExecListenerClass
* changeExecListenerPropertiesFile

As part of this change the ChangeExecListener can have one of several constructors, which will be determined at run time using some reflection.

ChangeExecListener()
ChangeExecListener(Database db, Properties props)
ChangeExecListener(Database db)
ChangeExecListener(Properties props, Database db)
ChangeExecListener(Properties props)

I also added a default impl of the ChangeExecListener which means you can override it and just implement the methods you need.

liquibase.changelog.visitor.ChangeExecListenerAdaptor

https://github.com/liquibase/liquibase/pull/505