Add support for proxy and basic authentication

[stdevCrow]

Feature description
In many educational centers, internet access is managed through a proxy, and it may require authentication.
Add support for proxies in requests, and also authentication (basic-auth at least)

Possible implementation
For testing purposes: Add the following lines after the imports statements:

http_proxy  = "http://user:password@10.8.6.41:3128"
https_proxy = "https://user:password@10.8.6.41:3128"
ftp_proxy   = "ftp://user:password@10.8.6.41:3128"

proxyDict = { 
              "http"  : http_proxy, 
              "https" : https_proxy, 
              "ftp"   : ftp_proxy
            }

Then add the proxies to each request, for example (see the proxies=proxyDict statement):

resource = session.get(url, verify=True, stream=True, headers=headers, proxies=proxyDict)

and

product_page = session.get("https://www.packtpub.com" + product_url[0], verify=True, headers=headers, proxies=proxyDict)

and so on...

Then add an optional command line option to handle this gracefully.

Notes

1. Sorry my english, it's not my mother tongue.
2. I don't code on python, so I don't fill that I can contribute, but I clone this repo and made the above modifications (thanks to stackoverflow) and it works as a charm through my authenticated proxy.