Task description support via forked orchestrator

I've made very simple and straightforward support for tasks listing and description. With [this pull request](https://github.com/orchestrator/orchestrator/pull/40) to orchestrator it's possible to pass a string as a last argument to `gulp.task`:
```javascript
gulp.task('build', function() {...}, 'Build my project');
```

This PR adds a `help` task by default, so running `gulp help` will produce the following output if `help` task is not overridden in gulpfile:
```shell
$ gulp help
[gulp] Using file ...
[gulp] Working directory changed to ...
[gulp] Running 'help'...

Available tasks:
 - help
 - build - Build my project

[gulp] Finished 'help' in 254 μs
```

This PR is only to start a discussion, i'll update docs and tests if we agree on this approach.