Refactor external table generation to separate library

[mrotteveel]

Being able to create (and maybe in the future: parse) external table files may also be useful to application programmers. Extract the generating (and, once available, parsing) of external table files to a separate library as part of this project (e.g. named ext-table-gen-core or ext-table-gen-api or something similar).

The library could then be published to Maven Central, and the ext-table-gen application would then use this library. Doing so will require some more thought on the exact boundaries and separation between the application and the library.

This should only be done when ext-table-gen has matured a bit more (received some additional features).