set grepprg=ag\ --vimgrep

function! Grep(...)
	return system(join([&grepprg] + [expandcmd(join(a:000, ' '))], ' '))
endfunction

command! -nargs=+ -complete=file_in_path -bar Grep  cgetexpr Grep(<f-args>)
command! -nargs=+ -complete=file_in_path -bar LGrep lgetexpr Grep(<f-args>)

cnoreabbrev <expr> grep  (getcmdtype() ==# ':' && getcmdline() ==# 'grep')  ? 'Grep'  : 'grep'
cnoreabbrev <expr> lgrep (getcmdtype() ==# ':' && getcmdline() ==# 'lgrep') ? 'LGrep' : 'lgrep'

augroup quickfix
	autocmd!
	autocmd QuickFixCmdPost cgetexpr cwindow
	autocmd QuickFixCmdPost lgetexpr lwindow
augroup END