Add dead time to config

Dead time is how long lsf keeps a file open after it hasn't been
written to.  By default, this is 24h.  This allows us to specify the
dead time we'd like to use.

Note: we need to have a "newer" version of LSF installed to use dead time, as the elasticsearch repo doesn't have the latest and greatest from the github.  logstash-forwarder/issues/200 explains its use and the need for a newer version.

If an older version gets this config, the field is simply ignored.

Tested on node, output looks like:

```
     {
       "paths": [ "file_path_here" ],
-      "fields": { "type": "lsf_type" }
+      "fields": { "type": "lsf_type" },
+      "dead time": "1h"
     }
```