Make expiryDelta in token.go configurable

[cgostuff]

For my application, I need an expiry delta higher than 10 seconds, which the value is currently hardcoded to in token.go. In order to use the builtin TokenSource or Client types, I have to manually hardcode the expiryDelta to a higher value. Having this configurable would provide a much cleaner way for me to use this library, and I think the same goes for some others (as can be seen in this PR https://github.com/golang/oauth2/pull/359, and in this issue https://github.com/golang/oauth2/issues/249).