Custom symbol mappings
**Which version of the plugin are you using?**
[ X ] Data Populator for Sketch
[ ] Data Populator for Adobe XD

**Is your feature request related to a problem? Please describe.**
I'd like to be able to use more Human-readable layer names in my overrides. It is also hard to use the plugin with libraries shared across teams if not everyone uses the plugin. 

**Describe the solution you'd like**
It would be great to be able to map the elements in my Json file to specific text layers in my symbols. Maybe in the JSON view in the plugin screen, there could be a selector next to each key-value pair to choose the text layer in the selected symbol

**Describe alternatives you've considered**
Even better would be a pickwhip style linking, like in Adobe After Effects, where you drag a line from each JSON key-value to a preview of the symbol they would be populated in. 

**Additional context**
I realize this is a pretty big ask, but just something I thought of while using your already great plugin. Thanks!
