Add in iPUZ support

[scouttyg]

This PR adds fairly robust support in for iPUZ files, along with some utility classes to hopefully detect other crossword filetype support in the future (e.g. jpz).

Will add more comments inline in the PR soon!

Relevant background on iPUZ: http://www.ipuz.org/

See also #246

Screenshots / Demo:

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[stevenhao]

thanks for contributing this change! do you have an .ipuz file handy for me to test with? where do people usually get them?

[scouttyg]

@stevenhao I've attached a few iPuz and Puz files for comparison if they help, along with their sources:

puz_and_ipuz_examples.zip

Sources:

1) https://crosshare.org/crosswords/GUNKcsoeee0gXGJNcICc/1847-british-literature-and-other-things 2) https://crosshare.org/crosswords/AsrgfbaS9P32f2x2w0eT/mama-mia 3) https://crosshare.org/crosswords/t4oUQn6DtLP9N62JtCdK/8-x-8 4) https://crosshare.org/crosswords/0kolHut2MOQWKxavia33/its-showtime

[stevenhao]

cool, solved one of the ipuz's and it looked fine!

[stevenhao]

thanks @scouttyg so much for helping with this. closes #246.