Harden geo support for CoreDNS

[maksymvavilov]

Work remaining after the initial PR in #365:

• Introduce sample db in the deployment (enables geo for non-local runs)
• Test functionality with a free db from maxmind
• Document geo functionality and provide samples

[maksymvavilov]

We decided to not provide a real database for the plugin. Clients will be expected to acquire their own database. Testing is done on the mock maxmind database. This is sufficient at the moment