add method to remove or ignore a stampField

[barthel]

It would be nice to ignore and/or remove the stampField to re-transform the class again.

[barthel]

If the transformation will be executed more than once the following exception was thrown:

[ERROR] Class xx.xxxxxx.xxxxx.xxxxx.XxxxxXxxxXxxxxxx$XXXXXxxXxXxxxxxxxx could not not be instrumented due to initialize FAILED.
javassist.bytecode.DuplicateMemberException: duplicate field: __TRANSFORMED_BY_JAVASSIST_MAVEN_PLUGIN__xx_xxxxxxxx_maven_plugin_javassist_transformer_VERInjectorClassTransformer
    at javassist.bytecode.ClassFile.testExistingField(ClassFile.java:602)
    at javassist.bytecode.ClassFile.addField(ClassFile.java:580)
    at javassist.CtClassType.addField(CtClassType.java:1265)
    at javassist.CtClass.addField(CtClass.java:1049)
    at com.github.drochetti.javassist.maven.ClassTransformer.applyStamp(ClassTransformer.java:211)