First merge request - Adding an action hook to the earnings report.

Wanting to create a hook that will let 3rd party developers perform additional calculations and echo the data under the earnings report graph; so am proposing new action hook called edd_reports_graph_additional_stats.

The appended arrays: dates, sales_data and earnings_data looks valuable for hook considerations. For example I could use $earnings_data and $sales_data to compute an estimated transaction fees paid for the given time and then echo the data into the report.

I am not sure how to docblock action hooks or I would have given it a try. Hope I'm submitting this the right way.