Add option to add custom attributes in appened image directive

[MichaelTSS]

Use case examples:

<div class="afkl-lazy-wrapper afkl-img-ratio-1-1"
afkl-lazy-image="{{profile.avatar_url}}"
afkl-lazy-image-options="{'background': false, imgAttrs: [{'onerror': 'this.src=\'/img/person-avatar.png\''}]}">
</div>

<div class="afkl-lazy-wrapper afkl-img-ratio-1-1"
afkl-lazy-image="{{profile.avatar_url}}"
afkl-lazy-image-options="{'background': false, imgAttrs: [{'title': '{{profile.title}}', 'data-id': '{{profile.id}}'}]}">
</div>

For backwards compatibility reasons, the alt option shout be kept but imgAttrs obviously support that.

[SquadraCorse]

Thanks MichaelTSS!