Allow choice of when to begin rendering modal

[jmpresso]

Currently, the modal begins to render on DOMContentLoaded. We have some code that wants to use the modal, but the modal won't be immediately launched. As so, we don't want to bother rendering it out initially, so we load the JavaScript with 'async'. This means the script may be loaded after the DomContentLoaded event is fired. As such, we'd prefer to have it run on the window.load event.

Can we have the option to choose to render it on the load event instead of DOMContentLoaded, or just disable the initial render and let us call a render function manually on load?