Add server option for client to support local bot API servers

lib/telegram/bot/client.rb

@@ -5,7 +5,8 @@ require 'httpclient'
 module Telegram
   module Bot
     class Client
-      URL_TEMPLATE = 'https://api.telegram.org/bot%<token>s/'.freeze
+      SERVER = 'https://api.telegram.org'.freeze
+      URL_TEMPLATE = '%<server>s/bot%<token>s/'.freeze
 
       autoload :TypedResponse, 'telegram/bot/client/typed_response'
       prepend Async
@@ -61,11 +62,11 @@ module Telegram
 
       attr_reader :client, :token, :username, :base_uri
 
-      def initialize(token = nil, username = nil, **options)
+      def initialize(token = nil, username = nil, server: SERVER, **options)
         @client = HTTPClient.new
         @token = token || options[:token]
         @username = username || options[:username]
-        @base_uri = format(URL_TEMPLATE, token: self.token)
+        @base_uri = format(URL_TEMPLATE, server: server, token: self.token)
       end
 
       def request(action, body = {})