Tensorflow version-pinning should be reflected in setup.py.

[sdenton4]

Hello! I maintain a repository which has a small dependency on TF-IO. We have to manually track the TF-IO and TF versions with the table in the readme, and update both at the same time.

However, there are sufficient tools to manage this dependency automatically! It seems like the 'extras_require' field can easily include the Tensorflow version pinning, and remove the need for downstream users to worry about the versioning.

[sdenton4]

Added a pull request here, for example:
#1957