Add sigdump

[sigdump](https://github.com/frsyuki/sigdump) created by @frsyuki is very useful gem. Why don't you require `sigdump` as default? 

This patch enables users to dump the internal status of ruby (fluentd) by just sending CONT signal to fluentd process pid like

```
$ kill -CONT <pid>
```

The dump is stored into `/tmp/sigdump-<pid>.log` file. 

Sigdump was very useful to debug why fluentd process stucked. See this as an example https://gist.github.com/sonots/e05d8fbcd7ab39e4a5f8.