Add sf-date item variant

[zcei]

While I was waiting for the review on #102 I was eager to learn a bit more and found that the date support was a nice hook for me to get to work with cargo features. So here we go, I hope I'm not taking it away from someone 😅

As I can't open a PR towards this repo targeting my previous branch, here's a link to get a proper diff of just the relevant commit:

https://github.com/zcei/sfv/compare/validate-bare-items...zcei:sfv:feat/date