Freeze with args

When I use `pip freeze` I am usually (95% of all times) interested to find out what version of one specific module is installed, rather than listing all packages at their current version. So, I end up doing something like e.g. `pip freeze | grep nump` all the time to find out what numpy version is installed. I propose to allow simply saying something like `pip freeze num sci` to only show versions of modules that match the given search strings.