bem server: Initialize bundle level targets

[arikon]

Read config from .bem/<something>.js (.bem/levels.js?), look for function-getter or glob variable, generate target for each bundle level present.

Config could be like this:

// regexp?
exports.bundleLevelsRegexp = /bundles.*?/;

// glob pattern?
exports.bundleLevelsGlob = 'bundles* pages*';

// function-getter (overriding default implementation)
exports.getBundleLevels = function() {
    return ['pages', 'widgets'];
};