Move get_total_customers method out of admin class into general purpose function

I'm putting together a little stats widget for our crowdfunding theme. I wanted a simple way of getting the number of customers. `EDD_Customer_Reports_Table` has `get_total_customers()` which is exactly what I want, but it's bound to that class. I'm proposing that it be moved into `user-functions.php` so that it can be used more broadly. 

Cheers, 
Eric