Output any eventual stderr from commands

The `compare` command-line tool operates in the sense that it outputs
it's info to stderr, which is very strange, but we would like to support
that. This way, people could do:

  compare = MiniMagick::Tool::Compare.new(false)

  # Do something with compare ...

  $stderr = StringIO.new
  compare.run
  output = $stderr.string
  $stderr = STDERR

It's not really pretty, but I think this is the best we can do, since
using stderr should be ultimatively fixed in ImageMagick.

Fixes #257

lib/mini_magick/shell.rb

@@ -26,6 +26,8 @@ def run(command)
         fail MiniMagick::Error, stderr
       end if @whiny
 
+      $stderr.print(stderr)
+
       stdout
     end
 

spec/lib/mini_magick/shell_spec.rb

@@ -32,6 +32,12 @@
       allow(subject).to receive(:execute).and_return(["stdout", "", 127])
       expect(subject.run(%W[foo])).to eq "stdout"
     end
+
+    it "prints to stderr output to $stderr in non-whiny mode" do
+      subject = described_class.new(false)
+      allow(subject).to receive(:execute).and_return(["", "stderr", 1])
+      expect { subject.run(%W[foo]) }.to output("stderr").to_stderr
+    end
   end
 
   describe "#execute" do