issue #49: add replace command to cli

for issue #49

This allows the cli to idempotently replace records in a set.  `--ttl` is optional, and if absent implies zone default.

Example (keeping in mind mock is not durable)
```
$ denominator -p mock record -z denominator.io. replace -n www1.myzone.com. -t A -d 1.1.1.1 -d 1.1.1.2 -d 1.1.1.3
;; in zone denominator.io. replacing rrset www1.myzone.com. A with values: [{address=1.1.1.1},{address=1.1.1.2},{address=1.1.1.3}]
;; ok
```