Added completion for _nl

Just a simple completion function for nl command (part of coreutils).

Feel free to discard, but please tell me why!
`\[numbering body\]` need this brackets?

```
Mandatory arguments to long options are mandatory for short options too.
  -b, --body-numbering=STYLE      use STYLE for numbering body lines
  -d, --section-delimiter=CC      use CC for separating logical pages
  -f, --footer-numbering=STYLE    use STYLE for numbering footer lines
               :
               :
```