Separate out IP lookup code into new repo

[mattolenik]

The IP lookup code is useful on its own, factor it into a new Go library and consume that library from this project.

Acceptance Criteria

• There exists a new GitHub repo for the new public IP package
• The new package also has a very simple command-line interface (no flags needed other than -version and -help)
• The command outputs the public IP to stdout and any errors or warnings to stderr