Add Reflector.Requirer to support custom struct field requiring

[funte]

Examples:

package main

import (
	"fmt"
	"reflect"

	"github.com/invopop/jsonschema"
)

type OptionBase struct {
	A int    `json:"a" jsonschema:"required"`
	B string `json:"b" jsonschema:"required"`
}

type OptionChild struct {
	*OptionBase

	C bool `json:"c" jsonschema:"required"`
}

func main() {
	r := jsonschema.Reflector{}
	r.RequiredFromJSONSchemaTags = true
	if s, err := r.Reflect(OptionBase{}).MarshalJSON(); err != nil {
		fmt.Println("failed to marshal OptionBase json schema, err=", err)
	} else {
		// Print OptionBase json schema, all struct fields should be required.
		fmt.Println(string(s))
	}

	// In OptionChild, cancel all requirings of OptionBase struct fields, just keep'C'.
	r.Requirer = func(f reflect.StructField, b bool) bool {
		return f.Name == "C"
	}
	if s, err := r.Reflect(OptionChild{}).MarshalJSON(); err != nil {
		fmt.Println("failed to marshal OptionChild json schema, err=", err)
	} else {
		// Print OptionChild json schema, only struct field 'C' should be required.
		fmt.Println(string(s))
	}
}

[samlown]

Would this also be achieved by #90?