Use norelativenumber instead of number

- This plugin is better for just modifying the relativenumber option since it only supports two states. The last commit removes modifications of the `number` option.
- Use `s:` prefixed vars for proper scope. `g:` means global which means other plugins can modify it. Furthermore, `g:mode` and `g:center` are not very unique names that might get used elsewhere.
- Fixed some inconsistent tab styling. I notice some other people did this too in their pull-requests.

If you have any questions or want help with the merging or anything let me know. I'm not very handy with vim script but I'll do my best.  Anyways, this plugin is a very good idea but it can be made much better if you merge the PR's currently against it.