Added color example

[danielflippance]

Added example of setting colors

[jtblin]

Thanks for the PR @danielflippance 👍

Would you mind also fixing the name of the function from getColor to chart-getColor as well to keep in the same commit?

It would have been great to add examples using the full object notation as well but i can do that later if you prefer: https://github.com/jtblin/angular-chart.js/blob/master/examples/app.js#L179