Add support for coercing string format=date query param

[langalex]

According to the spec, strings can have their format set to date in additon to the already-supported date-time.

When the validator sees such a string, it should coerce it into a Date.

I would send a PR if there is any interest to support this feature.

[basjanssen]

I've created a PR to add this #188 @geemus could you take a look?

[geemus]

Thanks @basjanssen for the assist.

Released in v2.3.1