Force email address on upvotes

Many upvoters don't leave their email address. It would be really useful if they did so we could contact them to take part in user research. It would be good if it were possible to configure our board so that email addresses are either mandatory for upvoters, or strongly encouraged (maybe it could be possible to customise the wording on the "enter your email address" pop-up for upvotes and on the email field for comments).