Add support for TOML config files

As much as I like YAML, [TOML](https://github.com/mojombo/toml) is where it's at. I'm sure @mojombo would agree. TOML is fully-featured and still very readable.

We'd need a Ruby parser. Based on [TOML's README under "Implementations"](https://github.com/mojombo/toml#implementations), we have the following four options:

- [toml](https://github.com/jm/toml) by @jm
- [toml-rb](https://github.com/eMancu/toml-rb) by @eMancu
- [toml2](https://github.com/charliesome/toml2) by @charliesome
- [tomlp](https://github.com/sandeepravi/tomlp) by @sandeepravi

My proposal is to add in support for TOML alongside YAML files, i.e. this is valid:

```bash
$ jekyll build --config _config.yml,overrides.toml
```

TOML and YAML would thus **both** be supported by Jekyll.

Thoughts?