Add support for setting/unsetting proxy in various software.

[Akarshit]

[clearnote01]

Let us add all the tools here that need proxy support (and it is possible to configure).
Add whatever you think needs to be supported.
(Let us just keep adding the tools here)
The list of the tools is

• git (supported now)
• pip/pip3
• npm
• mvn
• composer
• apt (supported already)
• youtube-dl (supports both socks and http from environment)
• heroku