Cloudflare API tokens are a new way to authenticate to Cloudflare API.
Unlike the Global API key, tokens can be given specific permissions to
only access parts of the API. See [1] for details.
[1] https://blog.cloudflare.com/api-tokens-general-availability/
This commit introduces a new credential for cloudflare called
`apitoken`, which is mutually exclusive with `apiuser` and `apikey`.
In order for DNSControl to work with this token, it should have the
right to read DNS zones and edit DNS records.
Closes#534
4e6d05b716