extractor: add support for log input via stdin

Now Extractor#run finishes just when no pipe, redirect, or any input file,
but is it overspec? Extractor#run should wait inputting for stdin?

lib/groonga/query-log/extractor.rb

@@ -42,17 +42,20 @@ def initialize
       end
 
       def run(*arguments)
+        log = ""
         begin
           log_paths = @option_parser.parse!(arguments)
         rescue OptionParser::ParseError
           raise(ArgumentError, $!.message)
         end
 
         if log_paths.empty?
-          raise(NoInputError, "Error: Please specify input log files.")
+          unless log_via_stdin?
+            raise(NoInputError, "Error: Please specify input log files.")
+          end
+          log = ARGF
         end
 
-        log = ""
         log_paths.each do |log_path|
           log << File.read(log_path)
         end
@@ -154,6 +157,18 @@ def target?(command)
 
         true
       end
+
+      def log_via_stdin?
+        input_with_pipe? or input_with_redirect?
+      end
+
+      def input_with_pipe?
+        File.pipe?($stdin)
+      end
+
+      def input_with_redirect?
+        not File.select([$stdin], [], [], 0).nil?
+      end
     end
   end
 end