Filter arguments passed to logs query for campaign backers

This adds a simple filter for the arguments passed to the logs query for retrieving a campaign's backers. 

I have two use-cases, both of which essentially boil down to the same thing:

1. WPML. I am creating a simple plugin that will unify a campaign's stats, so that language duplicates of a campaign all count towards the same goal. Not sure how you're handling this with your themes, but the cleanest way I can think of doing this is to filter the results of the backers query so that it automatically pulls in logs for the campaign's duplicates as well as the original campaign.

2. Parent or "umbrella" campaigns. This is essentially the same thing. One campaign, with many campaigns that don't raise funds for themselves, but raise funds for that one main campaign. 

Cheers, 
Eric