Proxy server support

[jaymzh]

This was migrated from: https://sourceforge.net/p/concordance/feature-requests/6/

It would be very nice to have proxy server support in concordance / congruity, as many people are sitting behind proxies. If that is not easily possible, it would be good mention in some readme file that proxies are currently not supported.

[swt2c]

This should at long last be solved by #40 as libcurl supports proxies using environment variables:
https://curl.se/libcurl/c/CURLOPT_PROXY.html