Enable bulk action for `helm-bibtex-open-pdf` and
`helm-bibtex-insert-key`.

In helm (and helm-bibtex), you can mark multiple candidates with
M-<SPACE> and do a bulk action on all selected candidates.

This commit enable bulk actions for these two actions. As for "Edit
Notes" and "Show Entry in Bibtex file", bulk action doesn't make much
sense to me or require more effort (auto open and layout multiple buffers?).