File extension for the rendered file(s) 

Hey, to clarify my use-case: 

- I use gulp + node-sass in combination with Rails, because I need the cache-busted files in my css, but for development speed I compile the css files with Libsass. To make Rails happy, I render the file with `.scss`extension. Totally optional feature, do nothing, and it works, as before. 

Would appreciate if you merge it. 
Thx for the great gulp plugin!