Show message when No input data provided

Thank you csvkit developers

lib/uplot/command.rb

@@ -35,8 +35,14 @@ module Uplot
         exit
       end
 
+      watchman = Thread.new do
+        sleep(1)
+        warn 'No input file or piped data provided. Waiting for standard input:'
+      end
+
       # Sometimes the input file does not end with a newline code.
       while (input = Kernel.gets(nil))
+        watchman.kill
         input.freeze
         @raw_inputs << input
         @data = Preprocessing.input(input, delimiter, headers, transpose)