Add simple enum validator (that can be any of type)

[evilandfox]

Hi! Thanks for your great tool)
I think "enum" validator will be very userful. At this time this is available for string validator. But, for example, I often use numeric enums in REST API as parameters.
I know that it can be made by using multiple validators. But that will be great to write like:

let schema = {
    status: { type: "enum", values: [null, 1, 2, "done"] }
}
v.validate({ status: "done" }, schema)

[icebob]

Good idea. Currently I have no time to implement it. If you think, you can create a PR. You should create a similar rule as string rule

[evilandfox]

Ok, I'll try to do so