Update help for `_completion` command

Currently when I do `program help _completion` I'm getting this:

```
 To enable BASH completion, run:

     eval `[program] _completion -g`.

 Or for an alias:

     eval `[program] _completion -g -p [alias]`.
```

This is very outdated information. Instead we should use compact version from https://github.com/stecman/symfony-console-completion#zero-config-use .

Related: #30
