[feature request] date filename for pixiv

[ghost]

I hope this possible, thank.

[mikf]

As mentioned in #232:

Use -K (or -j --range 1) together with a URL to get a list of all available metadata fields, and use those in filename and directory format strings. (Example)

Pixiv URLs all have a create_date field. It's only a string, but it very much looks like a date.

[mikf]

Pixiv URLs now also have a parsed version of create_date called date (80fdb11)