I decided it is not worth bumping version requirement just
for one DNS provider. I would prefer to stick to targeting
Go version available in latest Ubuntu LTS (Go 1.14 currently).
Support for a subset of libdns providers is added.
Some are enabled by default (assuming they are popular ones). AWS and Google Cloud SDKs take up extra 10 MiB of executable size.
Only filesystem storage is supported as of now.
Closes#3.