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).
Yeah it's likely overboard to have custom implementations of random. I think at the point which you'd seriously care about that you'd be using a more generalist library and generating your own colors.