Add ability to define resizing attributes
Hey there! I was wondering if there was any thought around potentially adding the ability to define the resizing options on Sketch Layers. Use case would be that I'm trying to build a Symbol library that can have flexible layouts at design-time.

Screenshot below of what I'm describing:

<img width="238" alt="screenshot 2017-07-12 16 36 05" src="https://user-images.githubusercontent.com/635300/28144433-4e64fbe0-6720-11e7-888e-8d72803a3437.png">
