Add sourceMap option.

In response to issue #12.

The `sourceMap` option, when enabled writes a source map of the concatenated file. I started from @kozy4324's https://github.com/kozy4324/grunt-concat-sourcemap. It has further options to store the file content in the map, inline the map in the concatenated file, and an option stored in the source map for the root directory of the source files.
I do like the nested option as well, as it keeps the source map options all bundled together, as they should be. The argument to keep it flat, if I remember right, was consistency with the majority of other tasks' options, which aren't nested.