add set layout option feature

[igorveremsky]

According to official AdminLTE layout documentation:

• Add feature to set layout option as layout property at AdminLteAsset.php
• Add feature to load extra assets (js, css, depends) according to specific layout
• Add feature to get html class by call static method \dmstr\helpers\AdminLteHelper::layoutHtmlClass()
• Add documentation to README.MD

TODO:

• [ ] feature to set more than one layout options
• [ ] some tests maybe (i`m bad with this)

Related issues:

• https://github.com/dmstr/yii2-adminlte-asset/issues/180

[schmunk42]

Looks good, is this BC-compatible by default?

[igorveremsky]

@schmunk42 by default layout option equals false so according to this expression this changes is BC compatible. Because, only method run() rewrited, other variables, methods, functions added.