Add support for manifest attributes introduced with java.lang.instrument

With J2SE 1.5 and the java.lang.instrument package, new manifest attributes have been introduced: Pre-Main-Class, Agent-Class, Boot-Class-Path, etc. (see http://java.sun.com/javase/6/docs/api/java/lang/instrument/package-summary.html for a complete list). It would be great if the maven-archiver could support these as well as it supports the very similar Main-Class and Class-Path manifest attributes. (Note, however, that the attributes introduced require the use of JARs; they are not completely independent of the archive format, so maybe maven-archiver is not a perfect fit.)