add metaevent filter

@jordansissel

Working with @jessedavis at Monitorama hackfest on this proof of concept for a metaevent filter which looks for sequences of events.  You can configure like:

```
filter {
  metaevent {
    tags => [ "foo" ]
    followed_by_tags => [ "bar" ]
    add_tag => [ "baz" ]
  }
}
```

Additional things we'd like to add if this looks like a good direction would be ```not_followed_by_tags``` and inlining the original events in the synthesized event.  Just wanted to get feedback early.