Convenience API for specifying the session options

[nagisa]

Currently in order to pass tensorflow session options you need to pass in a encoded protobuf of ConfigProto to the SessionOptions::set_config API.

While matching the API of e.g. Java bindings, this is also somewhat inconvenient, as the user is responsible for pulling in the ConfigProto schema into their own codebase and encoding the configuration they desire to use.

Having a (potentially as an optionally compiled feature) API to transparently encode and pass in the structure would be great.