Add cancelSubscription method

[jitendrapurohit]

User gets below warning on the cancel recur subscription page

Automatic cancellation is not supported for this payment processor. You or the contributor will need to manually cancel this recurring contribution using the payment processor website.

Stripe supports the cancellation of subcription. This PR adds this functionality to the extension.

Testing can be done using the below steps -

• Create a recur payment
• Click the cancel URL from the mail and cancel the subscription. No warning shown on this page and it also cancels the subscription from stripe site.

[mattwire]

ping @jmcclelland

[jmcclelland]

I think this is an important feature!

And, I think this is a duplicate of of #240, which has a long conversation in the pull request. I think the other PR handles possible errors when canceling an already canceled subscription and also it uses the more modern API to update the subscription id.

I'm commenting on the other PR since it seems a bit more active with one more thought about an upgrade script...