Refactor enb example. Use BEM methodology by default

One guy has written me about setting up `bla` for `enb` project. Our documentation didn't work for him because a trick with [adding `enb` section in a `package.json`](https://github.com/baby-loris/bla/blob/master/examples/frontend/enb/package.json#L14-L21) works only for `BEViS` projects.

So I rewrote the example for `BEM` projects and added [a special section for `BEViS`-like projects](https://github.com/baby-loris/bla/tree/enb/examples/frontend/enb#bevis).

Harsh reality shows us that most of people use `BEM` approach for their project. Only our team use `BEViS` :)