Solve 404 error when background-image is not a valid URL (bootstrap)

Hi, I'm Tony Briceño from Caracas-Venezuela.

First I want to congratulate, this plugin is very awsome and clear to read ( *thanks to that I could solve this issue in less than an hour* ).

I had a problem when include a bootstrap class " **.btn** ", because they use `linear-gradient` in the `background-image` property ( *really dont know why they do that, perhaps a CSS hack or something* ), so because of this your plugin request the image with the value of `background-image` causing an **404 ERROR**.

So I add a function wich validate an URL `function validateURL(textval) {...}`, and change the conditional from  `elementChildren.forEach(function(child) {...}`.

I leave commented the console.log's I used and the previous conditional.

Regards.

Att. [@tonybp18](https://twitter.com/tonybp18)