code: def endpoint(self, endpt, argdict=None):
          
          
          instanceargs = "{}&{}".format(queryjoin(key=self.key), queryjoin(self.args))
          if argdict:
              localargs = queryjoin(argdict)
              querystring = "{}&{}".format(instanceargs, localargs)
          else:    
              querystring = instanceargs
          return "{}?{}".format(self.ENDPOINTS[endpt], querystring)
docs: Construct API endpoint URLs using instance options in `self.args` 
          and local arguments passed to the function as a dictionary `argdict`.
          
          >>> api = BustimeAPI("BOGUSAPIKEY")
          >>> api.endpoint('VEHICLES') 
          'http://realtime.portauthority.org/bustime/api/v1/getvehicles?key=BOGUSAPIKEY&tmres=s&localestring=en_US'
          >>> api.endpoint('PREDICTION', dict(stpid=4123, rt="61C"))
          'http://realtime.portauthority.org/bustime/api/v1/getpredictions?key=BOGUSAPIKEY&tmres=s&localestring=en_US&format=json&rt=61C&stpid=4123'