Adding support for :set relativenumber / :set norelativenumber

Pull request to add a nice feature that I was missing on IdeaVim: relative line numbers.

By setting the relativenumber option, the editor shows the line numbers relative to the caret position, allowing users to quickly jump a few lines up or down to change some text or maybe yank it.

![screen shot 2014-03-09 at 11 08 28](https://f.cloud.github.com/assets/883380/2367678/d64d2526-a772-11e3-882a-5f113cf7a3ac.png)

With this feature you can just go 4j to go 4 lines down or 3k to go 3 lines up at a glance :)