Use `system()`, so that we don't have to redraw

`system()` just executes the shell command in the background, so we
don't need to redraw the screen in Terminal Vims. With redrawing the
screen blacks out for a split second, which doesn't look very nice :)
Merge pull request #23 from janko-m/use-system

Use `system()`, so that we don't have to redraw