Issue title: Overflow in Bootstrap Modals
 Issue body: No matter what I do, I can't get the dropdown to overflow the header/footer of a BS modal (without everything else overflowing too) - any guidance on that behavior?
 Comments: 
 Comment 0: Try setting the menu's `z-index` to a super-high number like 9999 (or higher).
 Comment 1: That didn't work either - sorry, didn't say that before.
 Comment 2: Do you mind putting up a sample page showing the issue?
 Comment 3: Issue was resolved by setting the modal-body's overflow-y to visible (from auto).
{'source': 'GithubIssues', 'repo_name_with_owner': 'thdoan/pretty-dropdowns', 'issue_id': 401978806, 'repo_stars': 110.0}