Allow ttl to be specified as duration string.

[captncraig]

Currently we expect DefaultTTL(3000) or TTL(100) with only numbers.

Would be cool if you could do DefaultTTL("4h") instead and defer to go's ParseDuration.

[tlimoncelli]

This should be handled at the JS layer, so that what makes it to the json file is pure.

helpers.js is going to need to call out to some Go code, perhaps a function called DURATION() which calls some go code like:

orig := "4h"   // The original string.
i, err := strconv.Atoi(orig)
if err != nil {
	d, err := time.ParseDuration(orig)
	if err != nil {
		panic(err)  // Do something wiser than panic
	}
	i = int(d.Seconds())
}
fmt.Printf("%d\n", i)

[tlimoncelli]

We added this a while ago. TTL("1h") is the same as TTL("3600") (and is more readable)

This is documented here: https://stackexchange.github.io/dnscontrol/js