Add downloader for AVCX

[afontenot]

Adds a downloader for AVCX (American Values Club Crosswords). These are popular crosswords from a variety of creators, see https://avxwords.com/about-us/.

This is a subscription-only crossword series, and requires authentication. This is handled in exactly the same way as NYT.

This downloader may not seem to serve an obvious purpose, given that AVCX emails subscribers an AcrossLite compatible .puz file for every new release. However, I'm thinking it will be useful for the following features:

• Automatic downloading of new crosswords, e.g. using a crontab.
• Would allow downstream software like Gnome Crosswords to fetch AVCX automatically.
• I do a few fixups on the .puz files and include difficulty metadata and the puzzle notes.
• Some crosswords are not available in .puz from the AVCX website, e.g. barred crosswords like https://avxwords.com/puzzles/1621. As these are available in JPZ I think it would be nice to do a very minimal translation into a close .puz approximation. (This is currently TODO.)