Add command to recursively print `help`

Right now, `--help` only goes one level deep. I think we should keep it that way, but it'd be nice to have another command that recursively prints out every command (that is, print the whole tree, including subcommands).

I could see it being useful for getting an idea of all the available commands, and also something someone could pipe to `grep` to find the command they're looking for.

## Examples
Right now:
`hanami --help`
```
Commands:
hanami assets [SUBCOMMAND]
hanami console # Starts Hanami console
hanami db [SUBCOMMAND]
...
```

Proposed:
`hanami --help-all` (Not sure what the best name would be)

```
Commands:
hanami assets precompile # Precompile assets for deployment
hanami console # Starts Hanami console
hanami db apply # Migrate, dump the SQL schema, and delete the migrations (experimental)
hanami db console # Starts a database console
hanami db create # Create the database (only for development/test)
hanami db drop # Drop the database (only for development/test)
hanami db migrate [VERSION] # Migrate the database
hanami db prepare # Drop, create, and migrate the database (only for development/test)
hanami db rollback [STEPS] # Rollback migrations
hanami db version # Print the current migrated version
...
```

## Resources

This request from @unrooty https://github.com/dry-rb/dry-cli/issues/55#issuecomment-580596940
This request from @kigster https://github.com/dry-rb/dry-cli/issues/55#issuecomment-520124037