Allow random colors in borderColor array

[flakmonkey]

Hi there. First of all, good work on this fun little project!

I'm not sure if you're looking for these kinds of changes, but they enabled my to do what I wanted to do with this project.

Border settings this enables:

borderColors: '#fff' // white border
borderColors: 'random',  // border of one random color
borderColors: ['random', '#fff'],  // random now works within this array
borderColors: ['random'],  // same as 2nd example

Of course, this breaks how borderColors: 'random' worked previously, since that would create a gradient of two random colors, and now it would only be a single solid-color border.

If it's preferable to keep that behavior, line 8 could be change to:

return colors.length < 2 ? colors.concat('random') : colors;

[webmatze]

Looks great. Thank you for your contribution to hyperborder. I will add your comments to the README and then publish a new version later.