[telegram] Allow specifying API Base URL

Feature:

  We can now pass a custom API Base URL to
  be used during making HTTP requests. This is useful in
  proxying requests, or running tests.

  In particular, this library is (almost) ready to
  be used with PWRTelegram API (http://pwrtelegram.xyz/).

README.md

@@ -137,6 +137,7 @@ Emits `message` when a message arrives.
 | [options.webHook.autoOpen] | <code>Boolean</code> | <code>true</code> | Open webHook immediately |
 | [options.onlyFirstMatch] | <code>Boolean</code> | <code>false</code> | Set to true to stop after first match. Otherwise, all regexps are executed |
 | [options.request] | <code>Object</code> |  | Options which will be added for all requests to telegram api. See https://github.com/request/request#requestoptions-callback for more information. |
+| [options.baseApiUrl] | <code>String</code> | <code>https://api.telegram.org</code> | API Base URl; useful for proxying and testing |
 
 <a name="TelegramBot+initPolling"></a>
 

src/telegram.js

@@ -51,12 +51,14 @@ class TelegramBot extends EventEmitter {
    * @param {Boolean} [options.webHook.autoOpen=true] Open webHook immediately
    * @param {Boolean} [options.onlyFirstMatch=false] Set to true to stop after first match. Otherwise, all regexps are executed
    * @param {Object} [options.request] Options which will be added for all requests to telegram api. See https://github.com/request/request#requestoptions-callback for more information.
+   * @param {String} [options.baseApiUrl=https://api.telegram.org] API Base URl; useful for proxying and testing
    * @see https://core.telegram.org/bots/api
    */
   constructor(token, options = {}) {
     super();
     this.token = token;
     this.options = options;
+    this.options.baseApiUrl = options.baseApiUrl || 'https://api.telegram.org';
     this._textRegexpCallbacks = [];
     this._onReplyToMessages = [];
 
@@ -169,11 +171,7 @@ class TelegramBot extends EventEmitter {
    * @see https://core.telegram.org/bots/api#making-requests
    */
   _buildURL(_path) {
-    return URL.format({
-      protocol: 'https',
-      host: 'api.telegram.org',
-      pathname: `/bot${this.token}/${_path}`
-    });
+    return `${this.options.baseApiUrl}/bot${this.token}/${_path}`;
   }
 
   /**
@@ -830,11 +828,7 @@ class TelegramBot extends EventEmitter {
    */
   getFileLink(fileId) {
     return this.getFile(fileId)
-      .then(resp => URL.format({
-        protocol: 'https',
-        host: 'api.telegram.org',
-        pathname: `/file/bot${this.token}/${resp.file_path}`
-      }));
+      .then(resp => `${this.options.baseApiUrl}/file/bot${this.token}/${resp.file_path}`);
   }
 
   /**