Support for creating a schema from existing JSON or map[string]interface{}/struct

[brocococonut]

As the title says, it'd be great to be able to create a schema on the fly from some existing JSON (or a map[string]interface{} or struct with JSON tags)

Some examples of how that might work:

// From struct
type Data struct {
	Name string `json:"name"`
	Key string `json:"key"`
}

data := Data{
	Name: "go",
	Key: "myKey",
}

schema := vjson.NewSchema(
	vjson.Struct(data)
)

// From a map
data := map[string]interface{}{
	"name": "go",
	"key": "myKey",
}

schema := vjson.NewSchema(
	vjson.Map(data)
)

// From a JSON string
data := `{
	"name": "go",
	"key": "myKey"
}`

schema := vjson.NewSchema(
	vjson.JSON(data)
)

All of which would be the equivalent in this case as:

schema := vjson.NewSchema(
	vjson.String("name").Required(),
	vjson.String("key").Required(),
)

[miladibra10]

It's an interesting idea.

Right now I'm a little busy and can not implement it right now.

It will be very nice if you contribute and implement this feature yourself. @brocococonut