Allow using prefix arg in `helm-next/previous-line' (#536)
Fix calculation of line number in mode-line.
* helm.el (helm-render-source): Use cl-loop and pass candidate number to `helm-insert-match'.
(helm-insert-match): New optional arg NUM.
(helm-output-filter--process-source): Use it.
(helm-candidate-number-at-point): New
(helm-display-mode-line): Use it instead of `line-number-mode'.
(helm--next-or-previous-line): Generic for *next/previous-line.
(helm-next-line, helm-previous-line): Use it.