Added ability to prefill customer email for anonymous checkouts

refs https://github.com/TryGhost/members.js/issues/10

- Allows passing an additional `customerEmail` value to our checkout creation API
- This value is used to pass `customer_email` option to stripe's checkout session - https://stripe.com/docs/api/checkout/sessions/create#create_checkout_session-customer_email.

The `customer_email` allows pre-filling the customer's email field in case of an anonymous checkout as customer doesn't exist already, and also ensures the stripe subscription is created with same email address as given by user during signup flow.

ghost/members-api/index.js

@@ -217,7 +217,8 @@ module.exports = function MembersApi({
 
         const sessionInfo = await stripe.createCheckoutSession(member, plan, {
             successUrl: req.body.successUrl,
-            cancelUrl: req.body.cancelUrl
+            cancelUrl: req.body.cancelUrl,
+            customerEmail: req.body.customerEmail
         });
 
         res.writeHead(200, {

ghost/members-api/lib/stripe/index.js

@@ -113,11 +113,13 @@ module.exports = class StripePaymentProcessor {
             customer = null;
         }
         const plan = this._plans.find(plan => plan.nickname === planName);
+        const customerEmail = (!customer && options.customerEmail) ? options.customerEmail : undefined;
         const session = await this._stripe.checkout.sessions.create({
             payment_method_types: ['card'],
             success_url: options.successUrl || this._checkoutSuccessUrl,
             cancel_url: options.cancelUrl || this._checkoutCancelUrl,
             customer: customer ? customer.id : undefined,
+            customer_email: customerEmail,
             subscription_data: {
                 items: [{
                     plan: plan.id