Context restoring

[MatthewZMSU]

Description

Implemented new downloader middleware for restoring contexts when they are destroyed. Added example spider. Added info about the middleware to README.

Restructured the project to have more than 1 middleware. Changed init.py in the corresponding way to provide backward compatibility.

Type of change

• [x] New feature (non-breaking change which adds functionality)
• [x] This change requires a documentation update

How Has This Been Tested?

I tested the middleware with several spuder on my PC.

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings