Support ALIAS records with Cloudflare (#89)

* simple facility for registering provider capabilities

* support for cloudflare. tests.

* js and docs

* docs

* generate

docs/_functions/domain/ALIAS.md

@@ -0,0 +1,25 @@
+---
+name: ALIAS
+parameters:
+  - name
+  - target
+  - modifiers...
+---
+
+ALIAS is a virtual record type that points a record at another record. It is analagous to a CNAME, but is usually resolved at request-time and served as an A record. Unlike CNAMEs, ALIAS records can be used at the zone apex (`@`)
+
+Different providers handle ALIAS records differently, and many do not support it at all. Attempting to use ALIAS records with a DNS provider type that does not support them will result in an error.
+
+The name should be the relative label for the domain.
+
+Target should be a string representing the target. If it is a single label we will assume it is a relative name on the current domain. If it contains *any* dots, it should be a fully qualified domain name, ending with a `.`.
+
+{% include startExample.html %}
+{% highlight js %}
+
+D("example.com", REGISTRAR, DnsProvider("CLOUDFLARE"),
+  ALIAS("@", "google.com."), // example.com -> google.com
+);
+
+{%endhighlight%}
+{% include endExample.html %}

docs/alias.md

@@ -0,0 +1,25 @@
+---
+layout: default
+---
+
+# ALIAS records
+
+ALIAS records are not widely standardized across DNS providers. Some (Route 53, DNSimple) have a native ALIAS record type. Others (Cloudflare) implement transparent CNAME flattening.
+
+DNSControl adds an ALIAS record type, and leaves it up to the provider implementation to handle it.
+
+A few notes:
+
+1. A provider must "opt-in" to supporting ALIAS records. When registering a provider, you specify which capabilities you support. Here is an example of how the 
+  cloudflare provider declares its support for aliases:
+
+```
+func init() {
+	providers.RegisterDomainServiceProviderType("CLOUDFLAREAPI", newCloudflare, providers.CanUseAlias)
+}
+```
+
+2. If you try to use ALIAS records, **all** dns providers for the domain must support ALIAS records. We do not want to serve inconsistent records across providers.
+3. CNAMEs at `@` are disallowed, but ALIAS is allowed. 
+4. Cloudflare does not have a native ALIAS type, but CNAMEs behave similarly. The Cloudflare provider "rewrites" ALIAS records to CNAME as it sees them. Other providers may not need this step.
+

docs/index.md

@@ -23,3 +23,4 @@ Dnscontrol is a platform for seamlessly managing your dns configuration across a
 
 - [Why CNAME/MX/NS targets require a trailing "dot"]({{site.github.url}}/why-the-dot)
 - [Writing Providers]({{site.github.url}}/writing-providers)
+- [ALIAS records in dnscontrol]({{site.github.url}}/alias)