helm-tags.el (helm-etags-select): don't force `execute action at once'

Provide a defcustom variable to control whether or not helm will be run
with `helm-execute-action-at-once-if-one'. It can be useful to not
execute the action at once if you actually want to jump to some
unrelated tag.
* helm-mode.el (helm--completion-in-region): Use buffer-substring-no-properties.
* README.md: update.
* helm-mode.el (helm--completion-in-region): Add space to input.
Merge pull request #370 from mgalgs/etags-exec-at-once

helm-tags.el (helm-etags-select): don't force `execute action at once'
* helm.el (helm-output-filter--collect-candidates): Issue #374 Don't collect nil candidates returned by filter-one-by-one.
* helm.el (helm-update-source-p): Issue #375 Fix backslash replacement.
* helm-mode.el (helm--completion-in-region): Fix filename completion in shell-mode.
* helm-mode.el (helm--completion-in-region): Fix error with args in symbol completion.
* helm-mode.el (helm--completion-in-region): Don't loose pcomplete completion on prog args.
* helm-mode.el: Simplify, now need to fix input for filenames.
* helm-mode.el (helm--completion-in-region): Keep fixing input insertion.
(helm-mode--in-file-completion-p): new.
* helm-mode.el (helm--completion-in-region): remove ./ and ../.
* helm-mode.el: Use cl-loop.
* helm-mode.el (helm--completion-in-region): exec-when-only-one.
* helm-mode.el (helm--completion-in-region): Not space after input in crm.
* helm-buffers.el (helm-buffer-match-major-mode): Issue #372 allow negation on modes.
* helm-buffers.el: Revert previous commit.
* helm-grep.el (helm-ack-grep-executable): New, Issue #377
* helm-buffers.el: Fix previous commit.