Add `hidden_aliases` option for commands decorator

### What component of Red (cog, command, API) would you like to see improvements on?

API

### Describe the enhancement you're suggesting.

Sometimes, we have a lot of aliases of a command in case someone forgets if it ends in an `s` or not, or how to spell something, etc. But it super clutters up the help to print every single alias. I would like to propose the following modification to the `@commands.command` decorator:

```python
@commands.command(aliases=['awakenings'], hidden_aliases=['awakening', 'awakening_help', 'awake_help', 'awoken_help', 'awokens_help'])
```

The hidden aliases would function identically to aliases but NOT be printed in the command documentation.