partial coloring of text: unicorn poop example
finish adding rainbow_text example, closes #697
adjust text size to fit in html docs
fixed naming and spacing issue, just for @pelson
ax is now an optional argument to rainbow_text

I hope @pelson and @tacaswell realize that this is just an example
gallery file when they were doing their code review
Merge pull request #2561 from ivanov/unicorn-fact

multi-colored text example