Allow us to configure whether the grok filter tags on failure

This pull request (hopefully, I'm relatively new to Ruby) adds a boolean config option to the grok filter called `tag_on_failure`.

Specifying true mimics what currently occurs. Specifying false will prevent the `_grokparsefailure` tag from being added to the message if not already present.

The reason for this is that I run the paths of our request logs through a series of grok filters to attempt to extract information from them and give the request a 'friendly' name. I'm not concerned about when the filter fails in this situation and would like to omit the `_grokparsefailure` tag (meaning I can spot actual failures).

There is a possibility that this request log tagging functionality could form a separate plugin but this seems a good starting point.