Exposing lengthOption and evaluatorList to extended classes

These methods make it easier to extend the `ThrowableProxyConverter`. 

For example, I extended the `ThrowableProxyConverter` to implement Nurkiewicz's stack trace filtering functionality (LBCLASSIC-325) here: https://github.com/mrietveld/jbpm/commit/6ee0481ce21c7eade8be23a6153ab21fffe2442c