Add Proxyability

[DannySotzny]

adds the possibility to use a proxy

[justdmitry]

Thanks for your contribution.

I replaced string proxyUrl with full-featured (credentials etc) IWebProxy in dedicated property, now you should create TelegramBot like this:

var bot = new TelegramBot(accessToken) {  WebProxy = new System.Net.WebProxy(...) };

[justdmitry]

Package ver. 3.7.50926 uploaded to NuGet.