Support entities/attributes in code listings

[fbolton]

If original DocBook uses entities in a code listing, the entities need to be converted to attributes and the syntax of the code listing needs to be modified to allow attributes in code. For example, you can use syntax like the following in AsciiDoc:

[subs="attributes"]
----
mvn archetype:generate \
  -DarchetypeCatalog={archetype-catalog} \
  -DarchetypeGroupId=org.jboss.fuse.fis.archetypes \
  -DarchetypeArtifactId=spring-boot-camel-archetype \
  -DarchetypeVersion={archetype-version}
----