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.
When @jmeas and I were looking at standardizing sourceMap options in grunt-contrib we had a number of discussions about what those options should be. I remember arguing a similar way to configure deeper options. I don't recall the specifics of the conversation but we decided we wanted to keep the options flat and to only those three. The primary tenet for that decision was for simplicity for users and the code in grunt-contrib.

We wrote down the outcome of our decisions in a repo https://github.com/jmeas/sourcemap-options#sourcemap-options. It looks like we didn't store the logs of some of the better discussions but did comment on the pros/cons of those options. We figured more advanced settings past the three would be put into a task for modifying sourcemaps after the fact. Reflecting on that, one of the elements for the decision may have been that having only two options in the deeper object seemed a little silly and possibly misleading since there would be no others.

If you wish to expand on this, it'd be great to hear your thoughts on it.