Modal bug squashing

# Changes
 - Fixes issue where an empty overlay would be shown in `NEUE.Modal.open()` was called with a non-existant selector.
 - Fixes an issue where a programmatically shown modal would not appear if modal container was not fully initialized. We now save a "queued" modal if `NEUE.Modal.open()` is called before the module is fully initialized.

For review: @DoSomething/front-end