Plugins are sometimes being registered twice

[VizuaaLOG]

Versions
BulmaJS: 0.7.0 (unreleased) and prior
Browser: Chrome (latest version)
Operating system: Mac OS (Latest version)

Describe the bug
If a plugin is initialised through javascript and it creates and inserts HTML straight away (i.e. creating a message and showing it immediately) when the traverseDOM method is called within the core. It will still create a new plugin instance for the element, even though it already has an instance associated with it.

To Reproduce
Steps to reproduce the behaviour:

1. Create a message element right after loading in the JS file and show the message immediately
2. In debug tools, the events are registered twice, and closing the modal will cause the second event listener to throw an error (as the first instance has already dealt with the event and deleted the HTML)

Expected behavior
The traverseDOM method should be able to identify an element that has already been initialised with a plugin.