Implement the ability to pass a block to encode_io and encode

This allows a library user to easily customize the representation of an
emoji (mapping it to a custom image for example)

lib/rumoji.rb

@@ -8,7 +8,15 @@ module Rumoji
 
   # Transform emoji into its cheat-sheet code
   def encode(str)
-    str.gsub(Emoji::ALL_REGEXP) { |match| (emoji = Emoji.find_by_string(match)) && emoji.code || match }
+    str.gsub(Emoji::ALL_REGEXP) do |match|
+      if emoji = Emoji.find_by_string(match)
+        if block_given?
+          yield emoji
+        else
+          emoji.code || match
+        end
+      end
+    end
   end
 
   # Transform a cheat-sheet code into an Emoji
@@ -16,9 +24,9 @@ module Rumoji
     str.gsub(/:([^\s:]?[\w-]+):/) { |match| (Emoji.find($1) || match).to_s }
   end
 
-  def encode_io(readable, writeable=StringIO.new(""))
+  def encode_io(readable, writeable=StringIO.new(""), &block)
     readable.each_line do |line|
-      writeable.write encode(line)
+      writeable.write encode(line, &block)
     end
     writeable
   end