Allow 'unique' attribute to be push/pulled through locomotive wagon

I encountered an interesting issue while trying to get a development environment set up for others. I created a model through the CMS and tried to pull it...and kept getting these errors on the unique property...I did some digging and managed to track down the problem to here as it's not a property on the content_field model. My suggestion would be the below - to add it as a property. I incremented the version in my own repo so that I could successfully pull and test that everything was working correctly. You could probably just cherry pick the first commit or enter a commit yourself.