Add `desc` method to describe commands: `desc 'my_command param'`

Add `desc` similar to Rake.

```ruby
desc '[top N] (persist|delivery) [last N (minute|hour|day|weeks)]'
hear /^(top\s+(\d+)\s+)?(persist|delivery)(\s+(.+))?$/i do |mdata|
  # ...
end
```

```bash
slack: help
[top N] (persist|delivery) [last N (minute|hour|day|weeks)]

slack: help -v
[top N] (persist|delivery) [last N (minute|hour|day|weeks)] => ^(top\s+(\d+)\s+)?(persist|delivery)(\s+(.+))?$
```