Implement mkcls or something similar to support IBM-4 models

The IBM-4 alignment model requires word classes. Giza generated the word classes using a clustering algorithm called [mkcls](https://github.com/clab/mkcls). Thot should provide a way to generate word classes. [ClusterCat](https://github.com/jonsafari/clustercat) is an alternative.