Add AsyncMake and AsyncGrep

Calling make and grep are the most common use cases for AsyncRun. How about documenting the following commands

command! -bang -nargs=* -complete=file -bar AsyncMake  AsyncRun<bang> -program=make -auto=make <args>
command! -bang -nargs=+ -complete=file -bar AsyncGrep  AsyncRun<bang> -program=grep -auto=grep <args>

that make make and grep work asynchronously ?

Perhaps its following variants

command! -bang -nargs=* -complete=file -bar AMake  silent noautocmd AsyncMake<bang> <args> | silent redraw! | copen | wincmd p
command! -bang -nargs=+ -complete=file -bar AGrep  silent noautocmd AsyncGrep<bang> <args> | silent redraw! | copen | wincmd p

that pop up the quickfix window, are welcome extensions.