Add much faster scrolling to counsel-grep

* counsel.el (counsel-grep-last-line): New defvar.
(counsel-grep): Set `counsel-grep-last-line' to nil.
(counsel-grep-action): Instead of going to `point-min' and
`forward-line' with a huge arg (e.g. to scroll 50K lines). Scroll
relative to the last position stored in `counsel-grep-last-line'.

This change resulted in a much smoother scrolling in a 50K line file
with 22K matches.