Move schemes to separate packages

[blond]

I think we should leave only flex scheme (#18) in this package.

• bem-walk-nested-scheme
• bem-walk-flat-scheme
• bem-walk-flex-scheme

Scheme package should provide walk and path method.

const nestedScheme = require('bem-nested-scheme');

const blockPath = nestedScheme.path({ block: 'button', elem: 'text' }); // button/__elem/

const info = {
    path: 'blocks/button/',
    depth: 1,
    naming: 'origin',
    level: 'blocks/'
};

const add = (file) => console.log(file);

nestedScheme.walk(info, add, err => console.log(err));

[blond]

We can also move the flex scheme and get it as npm dependency.