text: Title: Using pyppeteer behind a proxy
  Body: Hi,
  
  When we launch pyppeteer from behind a proxy for the first time, the downloader can't fetch the chromium zip.
  
  ```bash
  MaxRetryError: HTTPSConnectionPool(host='storage.googleapis.com', port=443): Max retries exceeded with url: /chromium-browser-snapshots/Linux_x64/588429/chrome-linux.zip (Caused by NewConnectionError('<urllib3.connection.HTTPSConnection object at 0x7ffa71bf6e90>: Failed to establish a new connection: [Errno -3] Temporary failure in name resolution'))
  ```
  
  By using an `urllib3.ProxyManager` instead of the `urllib3.PoolManager` and setting the proxy manually in https://github.com/pyppeteer/pyppeteer/blob/bc4b62555da445b252e0c9f1d7eefbd6dd8e52a0/pyppeteer/chromium_downloader.py#L82 the issue can be solved