Add support of SortFunc

[Kvaz1r]

Add support of SortFunc similar to MatchFunc.
It would be convenient when need to use some third-party libraries types.

Example:

package main

import (
	"fmt"
	"log"
	"github.com/shopspring/decimal"
	"github.com/timshannon/bolthold"
	bolt "go.etcd.io/bbolt"
)

type Item struct {
	Name  string `boltholdKey:"Name"`
	Score decimal.Decimal
}

func ErrorCheck(err error) {
	if err != nil {
		log.Fatal(err)
	}
}

func main() {
	data := []Item{
		{
			Name:  "Blue",
			Score: decimal.NewFromInt(7),
		},
		{
			Name:  "Red",
			Score: decimal.NewFromInt(5),
		},
		{
			Name:  "Green",
			Score: decimal.NewFromInt(15),
		},
		{
			Name:  "Yellow",
			Score: decimal.NewFromInt(9),
		},
		{
			Name:  "White",
			Score: decimal.NewFromInt(6),
		},
	}

	filename := "tempfile"
	store, err := bolthold.Open(filename, 0666, nil)
	defer store.Close()
	ErrorCheck(err)

	ErrorCheck(store.Bolt().Update(func(tx *bolt.Tx) error {
		for i := range data {
			err := store.TxUpsert(tx, data[i].Name, data[i])
			if err != nil {
				return err
			}
		}
		return nil
	}))

	var result []Item
	q := &bolthold.Query{}
	ErrorCheck(store.Find(&result, q.SortBy("Score")))

	for i := range result {
		fmt.Println(result[i].Name, result[i].Score)
	}
}

Output (items compared lexicographically):

Green 15
Red 5
White 6
Blue 7
Yellow 9