Handle scrollbar compensation

Usually, you would set `overflow: hidden` to body when the modal mounts. However, this causes a weird adjustment to the layout (because the scrollbar disappears). This mostly affects Windows/Linux users, and some OSX users that use a mouse (because it forces the scrollbar to appear).

As an example, compare [Semantic UI's Modal](http://semantic-ui.com/modules/modal.html) and [TWBS' Modal](http://getbootstrap.com/javascript/#modals). TWBS solved this by adding a `padding-right` to the `body` when the modal mounts.

\* I'm not sure whether this would make this project more opinionated. Moreover, I'm torn whether this belongs to the core functionality or to userland code.