Added report filters

Pull request against master, as requested (ref. [pull request 4747](https://github.com/woothemes/woocommerce/pull/4747)). Changes targeted to **WooCommerce 2.1**.

I added a few filters which will make it easier to alter the reports, if needed. Without such filters, the only way to alter the reports would be to override entire classes, such as `WC_Admin_Report` and `WC_Admin_Dashboard`.