Add field machine name to 'Fields' list in Index config

On the page admin/config/search/search_api/index/INDEX/fields you can select the fields to index for your custom index. In my instance I've 10 different content types, all containing different fields, but some would have the same label. Through the current UI, it's impossible to distinguish the similarly named fields based purely on their label - I have to look at the html source to see the field machine name.

To make this easier, I've added another column to the table to display the field machine name. Patch attached.