Build an arbitrary color from an input string

This allows to quickly build the same color from anything which can be
turned into a string.

Use case: you have a dataset for a graph, where the name of each set is
a defined string. You don't really care about the color generated, but
you still want something consistent from the same set, without fumbling
too much about making some kind of color. Here you are, just use
``colour.from_string(title)`` and you are done.