Attachable events on Dropzone init

I would like to suggest an easy adaptation to the Dropzone element on the` FilePicker` class. While working on a plugin to extend the functionality of Dropzone, it's come to my attention that I can only attach events to the Dropzone events. Such as `on added file` as per [documentation](http://www.dropzonejs.com/#event-list).

It would be nice to have an event that a plugin could attach to when Dropzone is initiated, as currently, even with the my plugin loading after admin.min.js, the Dropzone has still not init and as such, any modifications I do to it are then overwritten. Such a change should be easily implemented and could bring a whole new level of plugins.

A simpler approach albeit not as good would be to allow the previewTemplate variable (which creates the html for the dropzone) to be extended/overwritten.

I can provide more information upon request, either here or via the Slack.