Feature Request: Allow Placeholders in Overrides
Based off a conversation with @nitrada in #52...

The current behavior for using this plugin with symbols allows using the `{...}` placeholders within layer names (and layer content?) as such:

![image](https://cloud.githubusercontent.com/assets/8988697/24961499/f407447e-1f66-11e7-8231-8bf856d75361.png)

However, I'm in a scenario where I don't want to lock my symbols (nested symbols here) to a particular block of data. For example, let's say I have a grid and each column's cells are nested symbols (like normal content, a dropdown, a link, etc). Then, each cell should receive a different property of the JSON data **without locking the nested symbol to the given property**. In this case, instead of having to duplicate or detach my dropdown or content symbols, it would be great to be able to specify the placeholder (`{...}`) **within the override rather than the layer name** like so:

![image](https://cloud.githubusercontent.com/assets/8988697/24962103/dbfa2886-1f68-11e7-9f5b-bd04a7c3e943.png)

This approach allows certain symbols, like the dropdown and link, to be agnostic of the data they're being populated with. 

@nitrada I think this should sum it up pretty well but let me know if anything needs clarification. I looked through the `demo.sketch`, which did illuminate some of the features a bit better but unfortunately I still couldn't find a way to tackle this use case with the current feature set.