document global options in help task, close #1005

Looks like this:

```bash
$ lein help
...
Run `lein help $TASK` for details.

Global Options:
  -o             Run a task offline.
  -U             Run a task after forcing update of snapshots.
  -h, --help     Print this help.
  -v, --version  Print Leiningen's version.

See also: readme, faq, tutorial, news, sample, profiles, deploying, mixed-source, templates, and copying.
```