metadata: add CLI script for printing image metadata

Add a utility script for printing image metadata from the command line.

Usage: `bin/lsmetadata 1.jpg 2.jpg`

app/logical/media_file.rb

@@ -6,6 +6,8 @@
 # supported by a File.
 class MediaFile
   extend Memoist
+  include ActiveModel::Serializers::JSON
+
   attr_accessor :file
 
   # delegate all File methods to `file`.
@@ -168,5 +170,18 @@ class MediaFile
     nil
   end
 
+  def attributes
+    {
+      path: path,
+      width: width,
+      height: height,
+      file_ext: file_ext,
+      file_size: file_size,
+      md5: md5,
+      is_corrupt?: is_corrupt?,
+      metadata: metadata
+    }.stringify_keys
+  end
+
   memoize :file_ext, :file_size, :md5, :metadata
 end