Add HTTP & SOCKS5 proxy support

[Nerixyz]

Pull request checklist:

• CHANGELOG.md was updated, if applicable

Description

This PR adds an environment variable CHATTERINO2_PROXY_URL that can be used to specify a proxy (e.g. socks5://127.0.0.1:1080). The behaviour is similar to curl's CURLOPT_PROXY.

Partially fixes #450 since not all connections are proxied.

You can test this e.g. with mitmproxy.

Example Config

# ~/.mitmproxy/config.yml
listen_host: 127.0.0.1
mode: [regular] # or [socks5]
proxyauth: 'forsen:xqc' # optional
tcp_hosts:
  - 'irc.chat.twitch.tv' # don't interpret IRC as HTTP traffic

TODO Before Merge

• Document environment variable in Chatterino Wiki.

[github-actions]

clang-tidy made some suggestions

[pajlada]

HTTP proxies seem to be the most common, so since websocketpp supports HTTP proxies we should support that in this PR too