Add support for long descriptions

[AlexSkrypnyk]

We currently have usage which is shown when ahoy --help is called as a list of available commands and their usage information.

It would be great to be able to provide a description field with more expanded description of the commands. Such a description could then be available by ahoy [command] --help.

This would be in-line with how CLI commands provide short and long command help information.

$> ahoy build --help

Build project <-- Text from `usage`

Builds the container stack, front-end assets  <-- Text from `description` field.
and provisions a website.
A wrapper for several commands.

[ocean]

Good idea @AlexSkrypnyk - this was actually already part of the CLI library we're using, and just needed a small addition and some tests.

[ocean]

Implemented with #157 - not released yet though.