Add ability to write level config module as a function

[arikon]

We should add an ability to write level config module as a (async?) function, that should return (promised?) object of level extension.

This will also resolve possible problems with bem in different versions (project and libraries) interoperating in the same runtime.

modules.exports = function(BEM) {
    return {
        baseLevelPath: BEM.require.resolve('./levels/simple'),
        getTechs: function () {
            return {};
        }
    };
};

or

modules.exports = function(BEM) {
    var Q = BEM.require('q');
    return Q.resolve({
        baseLevelPath: BEM.require.resolve('./levels/simple'),
        getTechs: function () {
            return {};
        }
    });
};

Inspired by enb-make.js configs.

[eprev]