GitLab

Commit adds:
* Implementation of PowerDNS as a provider
* Tests for said implementation
* github.com/ffledgling/pdns-go, which provides go client bindings for
  PowerDNS's HTTP API, as a dependency
* "pdns" as an additional option for the `--provider` flag
* `--pdns-server` and `--pdns-api-key` as additional flags for PowerDNS
  specific configuration