Async sourcemap generator

Recently at workplace we changed our development process.
Now on every save we need all the code to be bundled (not minified), this means that on every save we expect the sourcemap to be generated again. With our current codebase this takes ~1.94s (and soon we're gonna need to create bundles for specific devices, so we expect an approximate wait of more than ~4.0s).

Considering that during development we don't need the sourcemap immediately available we thought that it would be nice to have the option to have grunt-contrib-concat create it asynchronously.

I think that this functionality could be useful also for someone else, therefore the pull request.