add `cmd` (command output expression).

mrblib/kernel.rb

 module Kernel
+  def self.`(cmd)
+    IO.popen(cmd) { |io| io.read }
+  end
+
   def open(file, *rest, &block)
     raise ArgumentError unless file.is_a?(String)
 

test/io.rb

@@ -232,6 +232,10 @@ assert('IO#fileno') do
   io.closed?
 end
 
+assert('`cmd`') do
+  assert_equal `echo foo`, "foo\n"
+end
+
 assert('IO TEST CLEANUP') do
   assert_nil MRubyIOTestUtil.io_test_cleanup
 end