it checks if the doujin exists.
returns boolean
it fetches the doujin information.
returns a promise
it sends the buffer as a pdf.
returns a promise
Usage:
const { Doujin } = require('../dist')
const doujin = new Doujin('https://nhentai.net/g/366327/')
doujin.validate().then(console.log)
doujin.fetch().then(console.log)
doujin.save().then(console.log)