Install ByteBuddy agent once per JVM (#74)

Move bytebuddy agent installation to a static block so
that the installation only happens once per JVM.

This ensures that the plugin works well with BUCK(and likewise) build
systems where multiple javac compilation task is started parallely in
the same JVM. Without this the JVM crashes undeterministically.

This will also work well in gradle build system where once javac task
runs per JVM

jabel-javac-plugin/src/main/java/com/github/bsideup/jabel/JabelCompilerPlugin.java

@@ -20,15 +20,12 @@
 import net.bytebuddy.pool.TypePool;
 import net.bytebuddy.utility.JavaModule;
 
-import java.lang.instrument.Instrumentation;
 import java.util.*;
 
 import static net.bytebuddy.matcher.ElementMatchers.*;
 
 public class JabelCompilerPlugin implements Plugin {
-
-    @Override
-    public void init(JavacTask task, String... args) {
+    static {
         Map<String, AsmVisitorWrapper> visitors = new HashMap<String, AsmVisitorWrapper>() {{
             // Disable the preview feature check
             AsmVisitorWrapper checkSourceLevelAdvice = Advice.to(CheckSourceLevelAdvice.class)
@@ -118,7 +115,10 @@ public void init(JavacTask task, String... args) {
                 Collections.emptySet(),
                 Collections.emptyMap()
         );
+    }
 
+    @Override
+    public void init(JavacTask task, String... args) {
         Context context = ((JavacTaskImpl) task).getContext();
         JavacMessages.instance(context).add(locale -> new ResourceBundle() {
             @Override