accept input file as cmdline argument

2021/day4/day4.py

@@ -94,4 +94,6 @@ def main(input_file):
 
 
 if __name__ == "__main__":
-    main("input.txt")
+    import sys
+    infile = sys.argv[1] if len(sys.argv) > 1 else "example.txt"
+    main(infile)