Add the command line command

src/fk_graph/cli.py

@@ -1,10 +1,36 @@
+from argparse import ArgumentParser
+
 from sqlalchemy import create_engine
 
 from fk_graph import setup_data, get_graph
 from fk_graph.plotly_functions import run_app
 
 def main():
-    engine = create_engine("sqlite+pysqlite:///:memory:")
-    setup_data(engine)
-    graph = get_graph(engine, "table_a", 1)
+    args = _parse_args()
+    if args.demo:
+        engine = create_engine("sqlite+pysqlite:///:memory:")
+        setup_data(engine)
+    elif args.connection_string:
+        engine = create_engine(args.connection_string)
+    graph = get_graph(engine, args.table, args.primary_key)
     run_app(graph)
+
+def _parse_args():
+    parser = ArgumentParser(
+        prog="fk-graph",
+        description="Visualise the graphs hidden within relational databases.",
+
+    )
+    parser.add_argument("--demo", action="store_true")
+    parser.add_argument("--connection-string")
+    parser.add_argument("--table", required=True)
+    parser.add_argument("--primary-key", required=True)
+    args = parser.parse_args()
+    if (
+        (not args.demo and args.connection_string is None)
+        or
+        (args.demo and args.connection_string is not None)
+    ):
+        parser.error(
+            "Exactly one of --demo and --connection-string should be used.")
+    return args