.vimrc

function! s:EnableRelativeNumber()
    set nonumber
    set relativenumber
endfunction

function! s:DisableRelativeNumber()
    set norelativenumber
    set number
endfunction

function! s:ToggleRelativeNumber()
    if &relativenumber == 1
        call <SID>DisableRelativeNumber()
    else
        call <SID>EnableRelativeNumber()
    endif
endfunction

autocmd InsertEnter * :call <SID>DisableRelativeNumber()
autocmd InsertLeave * :call <SID>EnableRelativeNumber()
nmap <silent> <leader>nl :call <SID>ToggleRelativeNumber()<CR>