Add themes

[adamschwartz]

It would be nice to have themes.

One would be the current solid bar with the blur peg. (Like nprogress.)

Another would be a MacOS-esque ribbed bar like that mentioned in Faster Progress Bars: Manipulating Perceived Duration with Visual Augmentations. (@zackbloom I'm still working on the styles for this.)

[adamschwartz]

@zackbloom I just went ahead and added mprogress-theme-ribbed.css.

It's now used in the demo but we'll probably want to make it a theme setting or something (or style or whatever).