Include commit_within config operation

This commit enables you to commit documents within a certain time (in ms) so that your solr core can be updated when a flush happens.

The issue I had is that my core was never updated after a flush, so no log results were showing.  This option allows you to define that commits happen regularly in order to make sure that the core will track changes accordingly.

Commit Within instructs solr to commit within a certain time, allowing you to alter the performance hit this operation has.

Info about this commitwithin is here:  http://wiki.apache.org/solr/CommitWithin
Solr's commitWithin operator is in milliseconds by default.

It might be confusing to someone who is familiar with solr, but not logstash.  Either way an easy fix.