Add gman_filter executable

Filters newline-separated email addresses from stdin

Example usage:

```
$ gman_filter < path/to/list/of/addresses.txt
```

To test:

```
$ gman_filter <(echo "
barry@dcpchicago.org
prof.obama@uchicago.edu
mr.senator@obama.senate.gov
president@whitehouse.gov
commander.in.chief@us.army.mil
")
```