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.
Merge pull request #370 from mgalgs/etags-exec-at-once

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