allow definining pre-gore-gulp dependency

[mcharytoniuk]

Allow to define dependencies that preceed gore-gulp tasks. For example:

gg({
  "baseDir": __dirname,
  "dependencies": [
    "my-task"
  ]
}).setup(gulp);

gulp.task("my-task", function () {
  gutil.log("Hello!");
});

Then:

$ gulp lint

Should output something similar to:

[18:26:39] Using gulpfile ~/gulpfile.js
[18:26:39] Starting 'my-task'...
[18:26:41] Finished 'my-task' after 0.85 s
[18:26:39] Starting 'lint'...
(...)
``