Merge pull request #1744 from doccano/enhancement/1598

Support workers option,

backend/cli.py

@@ -51,7 +51,7 @@ def load(self):
 
     options = {
         "bind": "%s:%s" % ("0.0.0.0", args.port),
-        "workers": number_of_workers(),
+        "workers": args.workers,
         "chdir": base,
         "capture_output": True,
         "loglevel": "debug",
@@ -134,6 +134,7 @@ def main():
     # Create a parser for web server.
     parser_server = subparsers.add_parser("webserver", help="see `webserver -h`")
     parser_server.add_argument("--port", type=int, default=8000, help="port number")
+    parser_server.add_argument("--workers", type=int, default=number_of_workers(), help="the number of workers")
     parser_server.set_defaults(handler=command_run_webserver)
 
     # Create a parser for task queue.