Add the --depedents option to the show command.

The show command should be able to list every installed package that depends on the packages passed as argument. 

```
$ pip show --dependents requests
---
Name: requests
Version: 0.14.2
Location: /path/to/requests/
Requires: 
Required by(1): httpie
```

This may be useful if you're going to uninstall some packages and don't want to break nothing.