Derive JsonSchema for commonly used types about schemars

[GREsau]

I'd like to have a general way of allowing derivations of JsonSchema for types in remote crates, which would probably be done with a #[schema(with = "TypeInMyCrate") attribute (where TypeInMyCrate implements JsonSchema), so that it can easily work with serde's remote deriving feature.

That said, for particularly popular crates like chrono, I think it's reasonable to add implementations of some types directly in schemars under a feature flag.

And JsonSchema will definitely be deriveable for tuple structs at some point, it's just not something I've gotten around to implementing yet

[GREsau]

In v0.1.8 you can now derive JsonSchema for tuple structs, and all serializable chrono types (Weekday, NaiveDate, NaiveDateTime, NaiveTime and DateTime) now have JsonSchema implementations under a feature flag.

schemars = { version = "0.1.8", features = ["chrono"] }