Document global CLI options in `grunt help`

[arithmetric]

It's not clear how to add help text for CLI options, like --timer, in the grunt help display powered by grunt-available-tasks.

We need to explore how to document options like this in the help text displayed.

[grayside]

This has narrower scope than #27, will rez and clarify that later.

I did some digging, and there's no graceful option for this right now.

1. Replicate help content via console.log() et al near the top of Gruntfile.js.
2. Using the included but unused grunt events api, fire an event at start of task dispatch that identifies the running operation. This allows a fancier version of (1)
3. As (1) or (2), but we parse the content out of the help content somehow.
4. The open issue in the availabletasks queue points to one of several active issues in which finer control of the grunt -h output is proposed.

Overall, I think we should use (1) and tell the user to check grunt -h. We may be dropping the availabletasks plugin for v0.5.

I'll come back later with issue links.