Address a possible pitfall related to config file

[demacdonald]

Currently iterate over the fields provided in the config ("error", "debug", etc) for colors.

Would be better if we iterated over the known set of functions we support instead.

In the case someone adds additional fields in the config that do not belong there we should not be creating additional chalk functions.

For example:

{
    "colors": {
        "logs": {
            "default": ["red"],
            "error": ["bgRed", "cyan"]
            "a": ["bgRed", "cyan"]
            "b": ["bgRed", "cyan"]
            "c": ["bgRed", "cyan"]
            "d": ["bgRed", "cyan"]
            "e": ["bgRed", "cyan"]
            "f": ["bgRed", "cyan"]
            "g": ["bgRed", "cyan"]
            "h": ["bgRed", "cyan"]
            "i": ["bgRed", "cyan"]
        }
    }
}

In above example, we are currently building chalk functions for each of them even though we will never use them.