Underneath mode

First of all, this is really **NOT** a big deal :)
The "Underneath mode" is just another way to add the backlight to the current line, which is using NSLayoutManager's `addTemporaryAttribute:value:forCharacterRange: ` method to add a background color to the current line.

The effect is just a little different though. You can check the attached image for the comparison.
The upper one is using the original way(a overlaid view), while the under one is using the so-called "underneath mode". Both backlight colors are using 0.2 alpha value.

*Please note the red parts. The color composition is a little different according to the position of the parts with alpha channel.*
![comparison](https://cloud.githubusercontent.com/assets/788863/5342350/94a3ff5e-7f40-11e4-818b-c92b4f0c806e.png)


I think the pros and cons of the "Underneath mode" are:
* Pros
  - Using `NSLayoutManager`'s method to render the background color is simpler and maybe faster than adding a subview to the editor's textview.
  - The color composition is a little clearer especially when using a dark theme with the backlight color of a little bigger alpha value(like > 0.2).
* Cons
  - We'll lose the "Stroke" and "Round corner" effect because what `NSLayoutManager` can do is just change the background color. (That's why I add this mode as another option of the original one. Let the user choose the appropriate one)