[vim] add easy cycling through numbering modes

Adapted from:
    https://raw.githubusercontent.com/wincent/wincent/master/roles/dotfiles/files/.vim/autoload/wincent/mappings/leader.vim

vim/vimrc

@@ -126,6 +126,22 @@ set nocompatible
     syntax enable
 " }}}
 
+" Functions {{{
+    " Cycle through relativenumber + number, number (only), and no numbering.
+    function! VimrcCycleNumbers() abort
+      if exists('+relativenumber')
+        execute {
+              \ '00': 'set relativenumber   | set number',
+              \ '01': 'set norelativenumber | set number',
+              \ '10': 'set norelativenumber | set nonumber',
+              \ '11': 'set norelativenumber | set number' }[&number . &relativenumber]
+      else
+        set number! " No relative numbering, just toggle numbers on and off.
+      endif
+    endfunction
+
+" }}}"
+
 " Mappings {{{
     let mapleader = "\<space>"
     let maplocalleader = ","
@@ -172,6 +188,10 @@ set nocompatible
 
     " fix whitespace
     nnoremap <leader>w :FixWhitespace<cr>
+
+    " cycle through line numbering modes
+    nnoremap <silent> <leader>r :call VimrcCycleNumbers()<CR>
+
 " }}}"
 
 augroup vimrc " {{{