Here is a simple extension of ocropus.  It's built either as a static executable with the
new component linked in, or as a shared library that can be linked dynamically into OCRopus.

You can invoke it like this:

    binarizer=MyThresholder ocrothresh threshold test.jpg out.png

For dynamic loading, you can use the actual ocropus command (this requires a recent version):

    extension=./ocrothresh.so binarizer=MyThresholder ocropus threshold test.jpg out.png

Dynamic loading will be enhanced so that OCRopus can load multiple dynamic extensions
in the future.
