completely separate the generated interfaces from manual ones and completely separate the manual tags from the generated ones. Also wipe the directories for code generation before code generation (#167)

code_gen/src/main/java/j2html_codegen/GeneratorUtil.java

@@ -0,0 +1,16 @@
+package j2html_codegen;
+
+import java.io.File;
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+
+public final class GeneratorUtil {
+
+    public static final void deleteAllFilesInDir(final Path dir) throws IOException {
+        for(final File file : dir.toFile().listFiles()){
+            System.out.println("deleting " + file.toPath());
+            Files.delete(file.toPath());
+        }
+    }
+}