support Cloudflare in addition to Dyn

[frankbraun]

The commands ssotpub createpkg, ssotpub refresh, and ssotpub signhead allow to automatically publish changes to DNS if Dyn is running the DNS server.

The same should be possible with Cloudflare. In order to do that the following needs to be done:

• Add option `ssotpub createpkg -cloudflare.
• Extend functionality accordingly for all three commands mentioned above.
• Create package util/cloudflare similar to https://godoc.org/github.com/frankbraun/codechain/util/dyn.
• Change the specification and implementation of https://godoc.org/github.com/frankbraun/codechain/ssot accordingly.

https://api.cloudflare.com/ documents the Cloudflare API.