Add CSV export in format expected by [WP-SeedBank](/meitar/wp-seedbank).

A similar project I maintain called the [WP-SeedBank](https://wordpress.org/plugins/wp-seedbank) is a WordPress plugin that can accept a CSV file to process multiple "seed exchange posts" at once. I figure exporting is a desirable data portability option regardless so I wrote you a patch that will export Virtual Seed Bank seed lists as CSV files in the format WP-SeedBank recognizes. Feel free to use as is or modify as you wish.