Provide helpers to download and build engines from lists

[remusao]

This change allows to start blocking ads with very little logic in Webextension, Electron and Puppeteer platforms! To achieve this, blockers abstraction now provide static methods to fetch pre-built engines from Cliqz's CDN or build them from scratch using lists of URLs to subscriptions. Here is how it looks like:

Webextension:

import { WebExtensionBlocker } from '@cliqz/adblocker-webextension';

WebExtensionBlocker.fromPrebuiltAdsAndTracking(fetch).then((blocker) => {
  blocker.enableBlockingInBrowser();
});

Electron:

import { session } from 'electron';
import fetch from 'cross-fetch'; // or 'node-fetch'

import { ElectronBlocker } from '@cliqz/adblocker-electron';

ElectronBlocker.fromPrebuiltAdsAndTracking(fetch).then((blocker) => {
  blocker.enableBlockingInSession(session.defaultSession);
});

Puppeteer:

import puppeteer from 'puppeteer';
import fetch from 'cross-fetch'; // or 'node-fetch'

import { PuppeteerBlocker } from '@cliqz/adblocker-puppeteer';

const browser = await puppeteer.launch();
const page = await browser.newPage();

PuppeteerBlocker.fromPrebuiltAdsAndTracking(fetch).then((blocker) => {
  blocker.enableBlockingInPage(page);
});

Fixes #216