Tasks in help output + descriptions

[paularmstrong]

Two ideas, unless I've completely missed some documentation:

1. List tasks in help output.

   moon task --help
   moon <project>: --help

2. Tasks should have some descriptions and help information. This would help describe to developers what the expected outcome of a task will be.

[milesj]

I'll look into this, but because of Rust's static nature, I'm not sure how possible it would be.

At minimum I can probably update moon task and moon project to include more info.