Nested Symbols support
Hi,
I have a button as a symbol with with padding on it. I have another symbol with multiple buttons on it. If I try to override the text, it doesn't get updated. 

Button Symbol
![screen shot 2018-02-05 at 6 03 33 pm](https://user-images.githubusercontent.com/52461/35838237-398f8af0-0a9f-11e8-9c99-d1126f654f2c.png)

Card Symbol
![screen shot 2018-02-05 at 6 03 54 pm](https://user-images.githubusercontent.com/52461/35838246-41c32c7c-0a9f-11e8-9a1d-343510ad6d85.png)

Symbol in use
![screen shot 2018-02-05 at 6 06 51 pm](https://user-images.githubusercontent.com/52461/35838300-7e63c56a-0a9f-11e8-9e91-742c2f1af30b.png)


Thanks