Do not require a minimum of 1 release branch

New feature motivation

When you start developing an application, you generally start by releasing alpha/beta/rc and do not have a "release branch". semantic-release doesn't allow this at the moment, as you need a minimum of 1 release branch.

At the moment, the way we work around it is by creating a branch that we just don't push to.

New feature description

Allow to have only pre-release branches.