add split_list plugin

This plugin allows you to split up an event into multiple events, similar to the `split` plugin, but instead of operating on newline-delimited text, it uses the data in a field that contains a list.

example config:

```
filter {
  split_list {
    source => my_list_of_items
    new_type => child
  }
}
```

This would take the event:

```ruby
{
    "my_list_of_items" => [
        { "event" => "one" },
        { "event" => "two" }
    ],
    "type" => "parent"
}
```

and create two new events like this:

```ruby
{
    "event" => "one",
    "type" => "child"
}
{
    "event" => "two",
    "type" => "child"
}```