* Stable comparison of metadata (#239)
Iterating over a map in Go never produces twice the same ordering.
Thus when comparing two metadata map with more than one key, the
`differ` is always finding differences.
To properly compare records metadata, we need to iterate the maps
in a deterministic way.
Signed-off-by: Brice Figureau <brice@daysofwonder.com>
* Support for Route53 ALIAS record type (#239)
Route53 ALIAS doesn't behave like a regular ALIAS, and is much more
limited as its target can only be some specific AWS resources or
another record in the same zone.
According to #239, this change adds a new directive R53_ALIAS which
implements this specific alias. This record type can only be used
with the Route53 provider.
This directive usage looks like this:
```js
D("example.com", REGISTRAR, DnsProvider("ROUTE53"),
R53_ALIAS("foo1", "A", "bar") // record in same zone
R53_ALIAS("foo2", "A",
"blahblah.elasticloadbalancing.us-west-1.amazonaws.com",
R53_ZONE('Z368ELLRRE2KJ0')) // ELB in us-west-1
```
Unfortunately, Route53 requires indicating the hosted zone id
where the target is defined (those are listed in AWS documentation,
see the R53_ALIAS documentation for links).
