Adds simple labkit plugin

[frauzufall]

These are preparations for operating Labkit without a user interface from e.g. from macros / scripts.

This PR..

• adds a command that takes an input image, a labeling image and returns the segmented output image
• adds a demo / test class

Example usage:

CommandModule result = ij.command().run(SimpleLabkitPlugin.class, true, "input", input, "labeling", labeling).get();
Img output = (Img) result.getOutput("output");

As discussed, it would maybe make more sense to have the classifier as an input parameter to the segmentation on multiple images without the need of a user interface.