Additional settings for y_colors

[Saucistophe]

I wanted to use simple vertical and horizontal gradients, without having to write a function. Would you accept a PR in which :

• y_colors can be an empty string, so that the output has an horizontal gradient based on x_colors;
• x_colors can be an empty string, so that the output has an horizontal gradient based on y_colors;
• both empty x_colors and y_colors will yield an error

[qrohlf]

@Saucistophe this seems reasonable. The only suggestion I have is that instead of using empty string, I'd rather have the user pass false to signify that they don't want to have an x or y component to the gradient.

Aside from that, I'd happily take that PR. Thanks for checking in!

[qrohlf]

closing this as resolved by #93, thanks @Saucistophe