merge draw_text into draw_label

[tbates]

Instead of maintaining draw_text and draw_label, it would be nice to just allow the (more capable) draw_label to take vectors of text, x, and y, check the lengths match, and call itself to execute each in turn.

One might then deprecate draw_text

[clauswilke]

A few comments:

1. In principle, I agree that having two separate functions is not desirable.

2. I'm happy to consider a pull request.

3. It would have to be verified that the more capable draw_text behaves exactly like the old one for regular text input. A lot of code is out there in the world using draw_text, and people shouldn't have to update all their figure-drawing code because of a minor improvement in the package.