BEM style string

by Songhn @songhn233

The Block, Element, Modifier methodology (BEM) is a popular naming convention for classes in CSS. For example, the block component would be represented as `btn`, element that depends upon the block would be represented as `btn__price`, modifier that changes the style of the block would be represented as `btn--big` or `btn__price--warning`. Implement `BEM` which generate string union from these three parameters. Where `B` is a string literal, `E` and `M` are string arrays (can be empty).