Option to subscribe to only "new major release" notifications

[jab]

libraries.io currently only allows subscribing to notifications for all new releases of a particular library. It would be helpful if users could subscribe to only new major version releases (as defined by semver), especially for libraries with very frequent minor or patch releases, where you currently have to choose between getting no notifications or getting a new release notification almost every single day.

Thanks for libraries.io, and thanks for your consideration of this enhancement proposal!