Provide some helper for merged bundle

[tadatuta]

In most of the cases merged bundle differ from other ones just in the step of getting declaration.
So it should be possible to write a module which would get all the techs from nodeConfig (or whatever?) and replace that step.

As a result the API should look like this:

config.nodes('*.bundles/merged', function (nodeConfig) {
    nodeConfig.addTechs (require('enb-bem-merged-bundle')(nodeConfig));
    nodeConfig.addTargets(['?.css', '?.js']);
});

[qfox]

In common, please don't use conditionals for configurations. Complexity shouldn't grow here...

[tadatuta]

for WIP please refer to https://github.com/bem/project-stub/compare/merged?expand=1