Add "qmemory" command (#553)

* Add qmemory command

* Update README, add test

* Redesign qmemory

* Add optional argument to show each worker's memory usage

* Add --workers argument to test_qmemory

django_q/management/commands/qmemory.py

@@ -0,0 +1,31 @@
+from django.core.management.base import BaseCommand
+from django.utils.translation import gettext as _
+
+from django_q.monitor import memory
+
+
+class Command(BaseCommand):
+    # Translators: help text for qmemory management command
+    help = _("Monitors Q Cluster memory usage")
+
+    def add_arguments(self, parser):
+        parser.add_argument(
+            "--run-once",
+            action="store_true",
+            dest="run_once",
+            default=False,
+            help="Run once and then stop.",
+        )
+        parser.add_argument(
+            "--workers",
+            action="store_true",
+            dest="workers",
+            default=False,
+            help="Show each worker's memory usage.",
+        )
+
+    def handle(self, *args, **options):
+        memory(
+            run_once=options.get("run_once", False),
+            workers=options.get("workers", False)
+        )