Change the type of .Site.Author from…

…`map[string]string` to `map[string]interface{}`.

This allows values other than `string` values to be saved to Author, such as:

    ```toml
    # config.toml
    …
    [Author]
      name = "Austin Ziegler"
      social-site = [ "Facebook", "Twitter", "GitHub" ]
    ```

My specific use-case is that I’m trying to make something work similar whether it’s specified in `.Params.Author` or in `.Site.Author` without introducing `.Site.Params.Author`.