use dirty db in a module

use dirty db in a module

first, create a mydb.js
```
var dirty = require('dirty');

let MyDB = class {

    constructor(dbname) {
        this.dbname = dbname
    }

    // Method
    open() {
        this.db = dirty(this.dbname)
        return new Promise((resolve, reject) => {
            this.db.on('load', function (length) {
                console.log('loaded!')
                resolve(length)
            })
        })
    }

    get_all_items() {
        return new Promise((resolve, reject) => {
            let ret = []
            this.db.forEach(function (key, val) {
                ret.push({ [key]: val })
            });
            resolve(ret)
        })
    }
}

exports.MyDB = MyDB

```
now, use it in another file:
```
"use strict"

const Mydb = require("./mydb").MyDB

var mydb = new Mydb('../data/items.db')

console.log('mydb', mydb);

(async function () {
    try {
        var rslt = await mydb.open()
        console.log('rslt', rslt)
        var all = await mydb.get_all_items()
        console.log(all)
    }
    catch (err) {
        console.log('error', err)
    }
}());

```