Scroll the quickfix window only if cursor is on the last line

if you set g:asyncrun_last to non-zero (it is zero by default):

let g:asyncrun_last = 1

Quickfix window will scroll only if the cursor is on the last line, just like output panel in vs.

But there are some downsides:

1. unlike :cbottom it is not native supported by vim (simulated in vimscript), checking cursor will have a lower performance, but most of time it's acceptable.
2. sometimes cursor blinks in a strange frequency, but not noticeable.

Due to these matters, g:asyncrun_last has been set to zero by default, you can turn it on manually. Hope Bram will support it native some day, and it will be unnecessary to simulated in vimscript any more.