Add experimental support for Java 13 class files

[Godin]

No description provided.

[Godin]

@marchof for the record here is end-to-end test:

$ mkdir src

$ cat <<END > src/Example.java
class Example {
  public static void main(String[] args) {
    System.out.println("Hello, Java 13!");
  }
}
END

$ java -version
openjdk version "13-ea" 2019-09-17
OpenJDK Runtime Environment (build 13-ea+4)
OpenJDK 64-Bit Server VM (build 13-ea+4, mixed mode, sharing)

$ javac --release 13 src/Example.java -d classes

$ javap -v -p classes/Example.class | grep major
  major version: 57

$ java -javaagent:jacoco/lib/jacocoagent.jar -cp classes Example
Hello, Java 13!

$ java -jar jacoco/lib/jacococli.jar classinfo classes/Example.class
  INST   BRAN   LINE   METH   CXTY   ELEMENT
     7      0      3      2      2   class 0x6ed8641f71bae73b Example

$ java -jar jacoco/lib/jacococli.jar instrument classes --dest instrumented
[INFO] 1 classes instrumented to /tmp/example/instrumented.

$ javap -v -p instrumented/Example.class | grep major
  major version: 57

$ java -cp instrumented:jacoco/lib/jacocoagent.jar Example
Hello, Java 13!

$ java -jar jacoco/lib/jacococli.jar execinfo jacoco.exec
[INFO] Loading exec file jacoco.exec.
CLASS ID         HITS/PROBES   CLASS NAME
Session "godin-laptop2-7ab9aa52": Wed Jan 23 01:47:30 CET 2019 - Wed Jan 23 01:47:31 CET 2019
6ed8641f71bae73b    1 of   2   Example
Session "godin-laptop2-26fcc118": Wed Jan 23 01:48:43 CET 2019 - Wed Jan 23 01:48:43 CET 2019
6ed8641f71bae73b    1 of   2   Example

$ java -jar jacoco/lib/jacococli.jar report jacoco.exec --classfiles classes 
[INFO] Loading execution data file /tmp/example/jacoco.exec.
[INFO] Analyzing 1 classes.