Allow disabling marked candidates.
* helm.el (helm-mark-all, helm-toggle-visible-mark): Don't mark when nomark present in source.
* helm-command.el (helm-M-x): Disable marking
* helm-elisp.el: disable marking in lisp completion.
* helm-eshell.el: disable marking in eshell completion and history.
* helm-sys.el: disable marking in top.
* helm-mode.el (helm-comp-read, helm-read-file-name): new keyword nomark.
(helm--completion-in-region): use it.
* helm-help.el: Document nomark attr.