Allow transformers to mark files for removal

To do this, they should return null rather than an entry. This fixes
GH-8.

Author: jamierocks

src/main/java/org/cadixdev/atlas/jar/JarFile.java

@@ -167,6 +167,10 @@ public void transform(final Path export, final JarEntryTransformer... transforme
                     // Transform the entry
                     for (final JarEntryTransformer transformer : transformers) {
                         entry = entry.accept(transformer);
+
+                        // If a transformer wants to remove an entry, it should return null.
+                        // TODO: document this in Bombe
+                        if (entry == null) return null;
                     }
 
                     // Write to jar