Generate eager and ndarray docs when releasing.

[dskkato]

Thank you for releasing v.0.18.0 of this crate. Also, the documentation for this crate is back in #336 and users can now check it at docs.rs.

https://docs.rs/tensorflow/0.18.0/tensorflow/index.html

However, I noticed that there is no additional features when generating the documentation. Also, the docs for https://tensorflow.github.io/rust/tensorflow/ are missing the eager and ndarray feature docs.

This PR resolves the above inconsistent docs in both docs.rs and github.io.

[adamcrume]

Thanks! This was an oversight. I wish that rustdoc would automatically indicate when additional features are required for certain items, but at least documenting it ourselves should be sufficient for now.