LOG4J2-589 CustomLevelPlugin initial version.
LOG4J2-589 Added logging, minor improvements.
LOG4J2-589 improved comments
LOG4J2-589 initial schema with outer CustomLevels container element
LOG4J2-589 updated schema without outer CustomLevels container element
LOG4J2-589 docs: user manual updates
LOG4J2-589 CustomLevel elements should be configured in a containing
CustomLevels element.

* Renamed CustomLevelPlugin to CustomLevelConfig
* Added method Configuration#getCustomLevels
* Added plugin CustomLevels to act as the CustomLevel container
* Updated manual page
* Updated schema
Merge remote-tracking branch 'origin/LOG4J2-589'
LOG4J2-589 added custom level configuration entry to changes.xml for
release notes