Add sourceMap option

Based on https://github.com/gruntjs/grunt-contrib-concat/pull/59, but:

* Sqaushed the fixup and in-between reverts (retained commit ownership to @mzgoddard).
* Rebased on latest master and resolved merge conflicts with other changes since.
* Fix incompatibility with node v0.8 and v0.10 by not using the `path.isAbsolute` method which so far is only released in Node v0.11 (Unstable).