Support of relative paths for levels tech

[blond]

Absolute paths:

nodeConfig.addTech([require('enb-bem/techs/levels'), { 
    levels: getLevels(config) 
}]);

function getLevels(config) {
    return [
        'common.blocks',
        'desktop.blocks'
    ].map(function (level) { 
        return config.resolvePath(level);
    });
}

Relative paths:

nodeConfig.addTech([require('enb-bem/techs/levels'), { 
    levels: ['common.blocks', 'desktop.blocks']
}]);