Add .ipuz export

[multimeric]

Motivation

Downstream uses of the puzzle data (for example, crossword apps) will want the puzzle in a structured format in order to import.

Currently there are 4 main well-defined crossword formats:

• .puz (binary and text subformats)
• .ipuz
• .jpuz
• .xpf (JSON and XML subformats)

Ideally genxword would support export in all of these formats, but I chose ipuz as the first option, because it has the best defined specification, and a Python validation library: https://github.com/svisser/ipuz.

Changes

• This introduces a new ipuz export format, triggered by 'z' in the invocation
• Using this will produce NAME.ipuz as an additional output
• I've also added a single test function located in a new file, test.py
• In addition, I had to add a dev extra to the setup file, such that you can install the test dependencies with pip install .[dev], and then pytest test.py

[riverrun]

This is great! Thanks for your help.

[riverrun]

Updated version 2.1.0 and pushed to pypi.