Allow dropdown-menu to to respect bounds of the viewport

[zbeat]

The default max-height of a dropdown-menu is 400px, as set here:
https://github.com/sukima/ember-cli-select-picker/blob/master/vendor/select-picker.css#L20

This is causing menus embedded towards the bottom of a page to spill off the viewable screen. As a temporary fix in our project, we can override this max-height to 300px. But in an ideal world, wouldn't we want the library to override the default max-height IF that would cause the menu to spill off the current viewport? Thoughts?

Of note:

1. It looks like bootstrap-select does this dynamically:
   https://github.com/silviomoreto/bootstrap-select/blob/a89ba9b56e6f7036afadbdf5c69718d79284c7dc/js/bootstrap-select.js#L862
2. The Bootstrap tooltip now has a viewport option, to accommodate this (e.g. https://getbootstrap.com/javascript/#tooltips):
   Add tooltip viewport option, respect bounds of the viewport twbs/bootstrap#11593

[sukima]

This is something to fix after PR #52 gets merged.