Hope to add support for http proxy

[xmbhjQAQ]

In some application environments, I can only connect to the telegram through the proxy. This means that the global proxy needs to be opened, which will affect the running of other programs

[creantan]

url_i := url.URL{}
url_proxy, err := url_i.Parse("http://127.0.0.1:1087")
bot, err := tgbotapi.NewBotAPIWithClient(os.Getenv("TELEGRAM_TOKEN"), tgbotapi.APIEndpoint, &http.Client{Transport: &http.Transport{Proxy: http.ProxyURL(url_proxy)}})

[zh4n7wm]

#38

This pull request support socks5 proxy