Allow users to publish directly to Sonatype

[jvican]

This is a feature request so that those users that dislike bintray can publish to sonatype instead.

@tpolecat has shown interest in releasing non-final releases to Sonatype instead of Bintray. Currently, we support releasing to Sonatype only final releases. https://gitter.im/scalacenter/sbt-release-early?at=594ae5bfcf9c13503cae3cca

Releasing to Sonatype would not require to define sbt-bintray settings and would bring in sbt-sonatype as a dependency. We need to reconcile both dependencies and avoid any clash that can happen from overriding settings/tasks.