Accept data on stdin

The most common way I've noticed many people do casual testing of API responses is with curl.
In particular some back-end devs really prefer the command line, which makes ajv-cli great for this purpose. Would it be possible for ajv-cli to take the data JSON from stdin so that

```CONSOLE
$ curl -X PUT "https://api.example.com/foos/506e3185e9c882d175a2d0cb0093d9f2" \
-H "Content-Type: application/json" \
--data '{"id":"506e3185e9c882d175a2d0cb0093d9f2","bar":"1234"}' \
| ajv validate -s foos-response.json
```

works? And likewise for `ajv test`. This of course is more to allow people to poke at things with minimal setup than something intended for a large scale test automation / CI system.