feat: modal:initialized callback + preload param + $.modal.get method. Fixes #279

[Zenoo]

• New event modal:initialized that runs once the modal content has been loaded.
• New preload param, defaults to false. When set to true, the modal doesn't open on initialization, and stays preloaded until we .open() it ourselves.
• New $.modal.get($elm) method that returns the modal instance of the linked element.

These changes should also fix #279.

[Zenoo]

As a side note, I couldn't get gulp to work with the version linked in package.json, so the minification comes from an automated VSCode tool.
Gulp 3 doesn't work with Node 11+, maybe upgrading it would be a good idea.