Add basic plugin implementation

.gitignore

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+node_modules
+.idea

.travis.yml

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+language: node_js
+sudo: false
+node_js:
+  - '8'
+  - '9'
+  - '10'
+  - '11'

README.md

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+[![Version](https://img.shields.io/npm/v/website-scraper-puppeteer.svg?style=flat)](https://www.npmjs.org/package/website-scraper-puppeteer)
+[![Downloads](https://img.shields.io/npm/dm/website-scraper-puppeteer.svg?style=flat)](https://www.npmjs.org/package/website-scraper-puppeteer)
+[![Build Status](https://travis-ci.org/website-scraper/website-scraper-puppeteer.svg?branch=master)](https://travis-ci.org/website-scraper/website-scraper-puppeteer)
+
 # website-scraper-puppeteer
-Plugin for website-scraper which returns html for dynamic websites using puppeteer
+Plugin for [website-scraper](https://github.com/website-scraper/node-website-scraper) which returns html for dynamic websites using [puppeteer](https://github.com/GoogleChrome/puppeteer)
+
+## Requirements
+* nodejs version >= 8
+* website-scraper version >= 4
+
+## Installation
+```sh
+npm install website-scraper website-scraper-puppeteer
+```
+
+## Usage
+```javascript
+const scrape = require('website-scraper');
+const PuppeteerPlugin = require('website-scraper-puppeteer');
+
+scrape({
+    urls: ['https://www.instagram.com/gopro/'],
+    directory: '/path/to/save',
+    plugins: [ new PuppeteerPlugin() ]
+});
+```
+
+## How it works
+It starts Chromium in headless mode which just opens page and waits until page is loaded.
+It is far from ideal because probably you need to wait until some resource is loaded or click some button or log in. Currently this module doesn't support such functionality.

index.js

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+const puppeteer = require('puppeteer');
+
+class PuppeteerPlugin {
+	apply(registerAction) {
+		let browser, page;
+
+		registerAction('beforeStart', async () => {
+			browser = await puppeteer.launch();
+			page = await browser.newPage();
+		});
+
+		registerAction('afterResponse', async ({response}) => {
+			const contentType = response.headers['content-type'];
+			const isHtml = contentType && contentType.split(';')[0] === 'text/html';
+			if (isHtml) {
+				const url = response.request.href;
+				await page.goto(url);
+				return page.content();
+			} else {
+				return response.body;
+			}
+		});
+
+		registerAction('afterFinish', () => browser.close());
+	}
+}
+
+module.exports = PuppeteerPlugin;