Merge pull request #656 from tuhdo/master

Add n/p keys for convenience
* helm-regexp.el: Fix typo (#657).
* helm-grep.el: Revert PR from issue (#656).
Such bindings prevent editing the *grep buffer.
* helm-regexp.el: Require helm-plugin (#644).
* helm-mode.el: Unquote helm-pattern (#662).
Unquote helm-pattern when it is added as candidate.
* helm-files.el: Don't append helm-files-save-history-extra-sources to
  completion sources.
(helm-files-save-file-name-history): Do it.
* helm-apt.el: Allow listing all installed packages.
* helm-apt.el (helm-apt-candidate-transformer): Revert previous change for this function.
* helm-apt.el: Make visible packages with arch (#667).
* helm-apt.el: Ensure we don't fail if one of the dpkg command returns more than one arg (#667).
* helm-apt.el: Remove one trailing white space, no code change.
* helm-files.el (helm-find-file-or-marked): Find backup files (#668).