Unity is .Net/Mono based, so you have access to a significant portion (but not all) of .Net's runtime to do these things, especially if you are only targeting the stand-alone player.

This would be a really simple way (there are about a bazillion other ways) of loading a configuration file using http://System.IO as suggested:

using http://System.IO;

public class GameConfig
{
    //TODO: create some variables to hold the parsed configuration values

    public void Load(string path)
    {
        string config = System.IO.File.ReadAllText(path);

        //TODO: parse the loaded configuration and set the variables to the values
        // contained in the config file.
    }
}

Files in Resources do get compiled into the runtime, so you'd want to use functions in System.IO.