Add custom stream support

[KristapsR]

Description

There are situations when we need to send response stream that is not an instance of node Readable stream and current way of checking for stream value instanceof Stream fails.
e.g. when using alternative module like readable-stream or some other cases

My usecase

import archiver from 'archiver'

const archive = archiver('zip')
const stream = ...
archive.append(stream, { name: 'archive.zip'})

res.body = archive

Solution

• Added tests to reproduce issue with mocked custom stream object that represents possible Readable stream implementation.
• Addopted code from is-stream

Tested also with this code

const archiver = require('archiver')
const Stream = require('stream')
const CustomStream = require('./test-helpers/stream.js')
const isReadableStream = require('./lib/is-readable-stream.js')

const stream = new Stream()
const archive = archiver('zip')
const customstream = new CustomStream.Readable()

console.log(stream instanceof Stream) // true
console.log(customstream instanceof Stream) // false
console.log(archive instanceof Stream) // false

console.log(isReadableStream(stream)) // true
console.log(isReadableStream(customstream)) // true
console.log(isReadableStream(archive)) // true