Use more Rust like interfaces for Config

[ronniec95]

Just some small changes

For gbdt::config::Config,

• a ConfigBuilder (following the builder pattern) that has a build() method to create a Config() object is more Rust like.
• The to_string() method is not necessary if you implement Display.
• The config.set_loss function takes str rather than an enum.

Are you open to PRs?

[dingelish]

Thanks! I’d happy to merge PRs!