Add custom properties setters

[DimitryDushkin]

We need this BEMJSON:

{  
   block: 'one',
   title: 'we need custom props'
}

Code:

BEMPRIV.decl('one', {
    init: function() {
        this._property('title', undefined, 'we need custom props');
    }
});

It would be great to write:

BEMPRIV.decl('one', {
    init: function() {
        this.title('we need custom props');
    }
});

[sbmaxx]

Fixed in https://github.com/sbmaxx/bempriv/tree/0.0.6