Add support for gulp-sourcemaps (from its creator)

Hello, sorry for adding another pull request on this. I got requests from different users of `gulp-sourcemaps` that are currently using my `gulp-concat` fork to send a pull request here, so they can use the normal plugin:
- I tried to leave your plugin as readable as possible
- all the concatenation and source mapping logic is in an external package
- full test coverage on the package: [concat-with-sourcemaps](https://github.com/floridoo/concat-with-sourcemaps)
I hope that you like this version so it can be merged soon.