Added support for broader use of sub html selectors.

[Livijn]

Solves #139. Could add some info to the docs if you want me to. Here is an example of how it works:

JS:

lightGallery(document.getElementById('kennel-page'), {
    selector: 'figure.image img',
    subHtmlSelector: 'figcaption',
    subHtmlSelectorFromParent: true,
});

Markup:

<figure class="image">
    <img data-src="URL" src="URL">
    <figcaption>Some html</figcaption>
</figure>

[cgpro]

@sachinchoolur is it possible to merge this? Would be a nice feature.

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