Default value for sprintf interpolation (Feature)

In a lot places, Logstash allows to reference fields with the _sprintf format_(https://www.elastic.co/guide/en/logstash/current/event-dependent-configuration.html#sprintf). Unfortunately if the referenced field does not exist, _sprintf_ defaults to leave the the reference untouched, which may lead to very unsatisfactory results. Therefore I suggest to allow a default value for the sprintf interpolation, e.g.

```
output {
file {
path => "/var/log/%{type:-default}.%{+yyyy.MM.dd.HH}"
}
}
```

As delimiter between the field reference and the default value I suggest `:-` (taken from bash, unlikely to exist in a Logstash event field name).