Some options for better file list sorting

- `g:netrw_sort_case_sensitive`
- `g:netrw_sort_dirs_first`
- `g:netrw_sort_dotfiles_first`

For the last `g:netrw_sort_dotfiles_first` option to work, a fixed version of `$VIMRUNTIME/autoload/netrw.vim` is required.
(See: https://code.google.com/p/vim/issues/detail?id=233)
Maybe I should split this PR and we should wait until that netrw.vim patch gets into Vim.