Use Apt-Get Behind a Socks5 Proxy

Nothing would be more frustrating than to install apt packages from sources outside of China, especially from PPA.
If you try to update the c++ compiler to g++-6 or g++-7 through

sudo add-apt-repository ppa:ubuntu-toolchain-r/test -y
sudo apt-get update
sudo apt-get install g++-7 -y
```
it may takes your hours to get it done.

But if you have a socks5 proxy out there, things will become better. The idea is to use `apt-get` behind a socks5 proxy.

`tsocks` and `proxychains` are common-used tools for this purpose. Here I choose `proxychains` over `tsocks` due to the DNS issue of `tsocks` mentioned by [this](https://serverfault.com/questions/754172/apt-get-maddeningly-doesnt-work-with-tsocks-proxy-set-up-with-ssh).

1. install `proxychains`

sudo apt-get install proxychains

1. setup for `proxychains` in `/etc/proxychains.conf`  
Since my socks5 proxy server is running at `127.0.0.1` through port `1080`, my setting is

socks5 127.0.0.1 1080

at the last line of `/etc/proxychains.conf`.

1. using `proxychaines` with `apt-get`
```bash
sudo proxychains apt-get update
sudo proxychains apt-get install g++-7 -y
```
Done in minutes!

You can also use the proxychains for other software as below:
```bash
sudo proxychains synaptic
proxychains julia

Enjoy!