Improve docs and errors regarding github_token

[jamescallumyoung]

github_token is listed as optional but required in some cases. When it required is not documented but in my usage I found it was needed at least when using Cloudflare DNS.

Additionally if github_token is required but not provided, an error message is provided. In this error message, the field is refered to as github-token (note the dash) instead of github_token. This can lead to ambiguity as to which field to use.

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Example error when github_token is not provided:

Error: Unhandled error: Error: Input required and not supplied: github-token

Example warning when github-token is provided instead of github_token:

Warning: Unexpected input(s) 'github-token', valid inputs are ['cid', 'dnslink_domain', 'cf_record_id', 'cf_zone_id', 'cf_auth_token', 'dnsimple_token', 'dnsimple_account_id', 'set_github_status', 'github_token']