Allow to output to a file

Hi,
I submit this pull request, which is 2-fold:

* Allows the task to produce an actual file which contents is the current revision, with the `out` property
* Remove the ability to augment the grunt config with the results

If you want to augment the grunt config with the results of `grunt-git-describe` for other tasks, you can still use templating once `grunt-git-describe` has been run, like

    <%= meta.version>