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'm not sure whether this is actually the case, but, nesting them might change the behaviour of overriding options for individual targets of the task.

They'll either be merged with the other options, or only with the internal defaults. Either of these may be nice depending on the use case. Neither is significantly better or worse. The key is in documenting the behaviour well.