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.
```
+      sourceMap: false,
+      sourceMapName: undefined,
+      sourceMapStyle: 'embed'
```
would it make sense to have 
```
options: {
  sourceMap: true
},
```
and
```
options: {
  sourceMap: {
    name: 'tmp/sourcemap3_embed_map.map',
    style: ...
  }
}