Added Description and SaucerHead features

[CrushedPixel]

This commit adds the OptionSetDescription function, which specifies a string to render in front of the progress bar.
It also adds the SaucerHead field to the Theme struct, which allows for the rightmost part of the saucer to be a different character than the others.
This is useful for arrow-like progress bars, as I use them in one of my projects:

Both of these changes are 100% backwards-compatible, so the behaviour of existing projects does not change - if the SaucerHead field is empty, the value of Saucer is used for the saucer head.

[schollz]

Thanks @CrushedPixel. This is great!
By the way, how did you get the colors?

[CrushedPixel]

I got the colors using mitchellh/colorstring in combination with k0kubun/go-ansi for Windows support.
I set the theme to:

Saucer:        colorstring.Color("[green]="),
SaucerHead:    colorstring.Color("[green]>"),

and render the progress bar into a bytes.Buffer which I later print using ansi.Print, which makes the color codes windows-compatible.