Added optional local version to package version

[nateagr]

Hi guys.

We are using Horovod/Gloo to distribute TensorFlow training on our Hadoop cluster. To do so, we build a wheel which fits our needs and which we publish on our private repository. When building the wheel, we would like to be able to add a local version to the public version so that:

• We clearly identify our local builds to public builds.
• We can build multiple time the same public version of Horovod, with different version of TensorFlow for example.
  In this PR, I follow recommendations of https://www.python.org/dev/peps/pep-0440/#public-version-identifiers
  What do you think ? Thanks.

[tgaddair]

LGTM!