Feature request: Support custom HTML element type

[SuperPat45]

It will be awesome if I can show a custom text message for not previewable file types like Google drive:

So a new option "customElement" could be added:

var element = $("<div id="unpreviewable">Unable to preview this item</div>");
BigPicture({
	el: e.target,
	customElement: element
})

or by id:

BigPicture({
	el: e.target,
	customElement: "unpreviewable"
})