Adding support for seed based colour generation

As discussed earlier I've introduced support for generating colours based on a seed.

A seed is a string of characters you can pass to the `make_colour()` function; the same string of characters will always return the same colour.

Example:

    // Will always return "#bf8172"
    Please.make_color({seed: 'foobar'});
    // Will always return ["#72bf9c", "#9572bf", "#bf9972"]
    Please.make_color({seed: 'GitHub', colors_returned: 3});

Note that if the colour generation code changes in a future version of the library, the colours generated for specific seeds might change (depends on the number of times the random functions are called).
I don't understand how is that better or more efficient than checking if `randomiser` is an instance of `RC4Random`, which is guaranteed to have a `random` function?