Allow links like auth to auto open in browser

Add method `open_link` to use mac `open` command line only if mac
Add method `is_mac` to determin if its a mac OS

lib/console_tweet/cli.rb

@@ -48,6 +48,7 @@ def get_access_token
       request_token = @client.request_token
       # ask the user to visit the auth url
       puts "To authenticate your client, visit the URL: #{request_token.authorize_url}"
+      open_link request_token.authorize_url
       # wait for the user to give us the PIN back
       print 'Enter PIN: '
       begin
@@ -230,6 +231,14 @@ def print_tweets(tweets)
       end
     end
 
+    def open_link url
+      exec "open #{url}" if is_mac?
+    end
+
+    def is_mac?
+      RUBY_PLATFORM.downcase.include?("darwin")
+    end
+
   end
 
 end