Add support for gulp-data

It would be handy to be able to pipe in options, such as commit messages, using [gulp-data](https://www.npmjs.com/package/gulp-data).

This would permit the following task for tagging versions:

```js
var gulp = require('gulp');
var bump = require('gulp-bump');
var git = require('gulp-git');
var data = require('gulp-data');

gulp.task('tag', function () {
  var version;
  
  //pipe the package.json through gulp-bump and save it
  return gulp.src('./package.json')
    .pipe(bump())
    .pipe(gulp.dest('./'))
    .pipe(data(function (file) {
      //parse the package.json and get the version number
      version = JSON.parse(file.contents.toString()).version;
      //construct the commit message from the version number
      return {message: 'Version ' + version};
    }))
    //git.commit will get the commit message from the gulp-data function above
    .pipe(git.commit())
    .on('end', function () {
      //tag the new version
      git.tag('v' + version, 'Version ' + version, function (err) {
        if (err) throw err;
      });
    });
});
```

Thanks!