Introduce support for alpha versions

Introduce support for alpha versions

An alpha version can be used to name a release while a previous one is still
being developed.

E.g. 2.9.0~alpha1 can be out while 2.8.0~rc1 is being worked on.

When a version is marked as alpha, it is allowed to have unreleased versions
before it in the NEWS file, in order to simplify merges of the NEWS file itself
between the two development branches.

Partially fixes Issue 448.

Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>